Some recipes (especially baking-type ones) are fussy -- the amount of liquid-type ingredients vs. the amount of dry-type ingredients can make a real difference. But (from my more-limited-than-some-on-your-friendslist experience) this recipe looks pretty flexible. I'm pretty sure that the exact amount of celery or green peppers won't make a real difference -- and that replacing, say, the peppers with mushrooms wouldn't matter, either. (1 scant cup? 1 overflowing cup? Go for it!) So start with a couple of peppers and a nice head of celery and see where it goes!

After all, m'dear, cooking for a 4-star restaurant might take special training, but homestyle cooking just takes practice. So, you're practicing. And you can bake bread, which is a lot more than some!