People are so nice here and share their knowledge and experience. I'm too new to help, but I hope it's a keeper!

It looks like fleabane to me. The first year it has just a rosette of leaves like what you currently have. The second year it has tiny pinkish-white daisies with fine petals and yellow centers.

I had the misfortune to get some in a 'wildflower mix' and it has seeded around like a weed ever since. I would recommend removing it.

Did you sow annual asters? They also look like those leaves.

Fleabane does cover a wide range of plants. ;-) Including aster. None which I have sown.

They grow on a place where I have sown two types of melon. But I saw them nowhere else in the garden. I hoped it was melon, but I doubted it, because I was told they are hard to grow in the Netherlands and I am just starting. So I suppose I can remove them and sow something else. A pity, but what I expected.

Or maybe I transplant them to a part of the garden where I do not mind to have flowers. Some versions look nice and all attract insects I think.