I don't think of barley aas interchangeable with rice. But making the dish with barley might be good, but very different.
Linda C

If using barley, I would cook the barley first, in a large amount of water. When tender but still chewy (if you cook it too soft, it will tend to dissolve somewhat and get mushy), I would rinse and drain it very well, cooking the vegetables and seasonings separately to toss together when done. Barley releases quite a bit of starch as it cooks, and ends up sticky like a badly cooked risotto. Rinsing and draining well counters this. In fact, after draining it well, I would probably spread it out in a thin layer on a flat pan to air dry for a while before tossing the other ingredients together with it. I use barley quite frequently in mixed grain salads, and this is the technique I use when doing so.

I think the barley would not go with the other flavors well.
To me it would ruin the dish you want to create. You will already be using frozen fish, stick with the rice.

Maybe I'm the standout, but I'd do it.

You don't say what KIND of barley you have. Whole barley - hulled, Scotch or pot barley, pearled barley, instant barley? You'll have to adjust cooking time according to each type of barley. I think you can successfully substitute barley in the recipe. I would even go so far as to say a mix of barley and lentils could also work.

-Grainlady

It is pearl barley and I do like the idea of lentils. I have a hard time with recipes as it is - deciding if they are good to me or not (first time using recipes, that is). Plus, barley, quinoa are pretty new to me. If I did keep it rice, the lentils might be a nice addition. I think I would add them separately and at the end (sound right?).