Usually people who are installing something that nice know how to do that better. Then, again, the fault here is to the drywallers as well as the countertop installers. Is this a new house? If so, the builder needs to fix that. If it is a remodel, the GC or remodeler is at fault. If you have to fix it and feel capable, you could do the following: 1. Tape the top trim mon the backsplash. Use blue painters tape. Then cover the countertop with brown craft paper. 2. Using a hand sanding pad, scuff the paint on the wall above the counter. 3. Using a wide(12" or 14" drywall knife--those look like a really wide putty knife), apply dry wall joint compound on the wall to fill the gaps. Don't try to fill a 1/2" gap the first time. Do it in two separate applications. You could use regular mud-let dry for at least 12 hours. Or use 'hot' mud, which can dry in a couple hours. 4. Sand smooth with a dry wall sanding pad and drywall sandpaper. 5. Prime and paint. 6. There will still be a small gap. Use a colored caulk to fill that gap.

Here's what our fabricator did: Before counter installation: this was an existing window and we didn't move it. The distance from the countertop to the bottom of the window opening would be less than an inch. Right after the counter and window sill were installed. Our fabricator laminated a piece of material of the thickness needed to the bottom of the sill - similar to what is done when making the counter edge. I think it would look right under each window as an extension of the sill and ending where the sill ends like ours does or under the whole group of windows. It may be easier to fabricate doing it the first way as it would be a very long thin piece for one piece to extend under all 5 windows. It looks like your filler will have to be thinner than your granite slabs so it may be difficult/expensive for the fabricator to make it extend much wider than the sill for the stacked appearance you want. Our fabricator did say that he was able to make thin pieces with our stone that can't be made with every stone when he made the thin pieces to trim our rangetop. He fabricated these pieces which are less than 1/2" high and ~3/4" wide (the slab thickness) and a bit over 2' long to fix a slight error they made when making the rangetop cut out. But I think the thin piece for the window sill wasn't as much of a concern for him because it was laminated onto the sill when it was made and didn't have to be carried around separately like the rangetop trim pieces.

I’ve only skimmed the comments, but I was just about to say the same thing as Chessie—that the range you’re describing where the sides of cooktop sit on top of the countertops next to them used to be known as drop-in style, but nowadays some slide-ins—like the dual-fuel Bosch mentioned above—also have cooktops that sit on top of the countertops, so the verbiage is a bit muddled these days...or maybe it’s my brain that’s muddled. :) Edited to add that as far as cleanability (Apple is telling me that’s not a word because it’s underlined in red dots & is suggesting it be changed to “clean ability,” but, whatever) goes, a conventional slide-in (i.e., the sides are unfinished, controls are in the front and the cooktop sides do not sit on top of the countertop) or freestanding range is about the same or maybe even better in that department: With a slide-in or freestanding, crud goes into the crevice and you do your best to keep it kinda clean until you slide it out during your annual or semi-annual deep cleaning. With a drop-in, crud goes underneath the overlying cooktop sides and then gets pushed into that crevice while you’re doing your best to keep it kinda clean until—wait!: You can’t slide it out; it’s on a freaking platform—well, mine is, anyway. :/ P.S. If anyone was able to pick up what I was laying down in the preceding gobbledygook, I’m impressed.

What did you end up using ? I am sick offgassing, outgassing and plastic, silicone, etc. The cheap filler kits look awful, IMO and are the big box stores' are expensive (probably because they often come with 5 pieces in a kit when all I want is 2). I' could see using a good wood strip, tailored to fit. With an induction stove it would be very difficult to get wood to burn. I will check etsy but curious where you landed.