Elastic Recipes: how to do it

I posted one of my favorite recipes on Sunday, for ratatouille, and I said it was elastic.  What I didn't say was how to stretch it up or down, and I thought it might be useful to do a little walk through of that in case anyone didn't catch what I meant.  If you already figured this part out, you're awesome, gold star.  If you didn't figure this out, you're still awesome, and here we go:

The main idea is to keep the proportions of the ingredients the same.  So in my ratatouille recipe, the first items in the ingredients list are all 1 lb each:  eggplant, squash, and so on.  If you wanted to make half as much ratatouille, you would go with half a pound of each of these, then a quarter pound of mushrooms later.  Whatever you do to the first ingredient, you do to each of the ingredients with a measurement.  Conversely, if five pounds of ratatouille isn't enough and you want to double the recipe, just double each of the weighed ingredients.  Two pounds of eggplant, two pounds of squash, a pound of mushrooms, et cetera.  

Can we just pause for a moment here?  Because five pounds is a lot of veggie.  Just wanted to make sure you're with me.  OK, fantastic.  Let's continue. 

That's the easy part, but it's not the only way you can fiddle with an elastic recipe.  For this one specifically, you can tweak the vegetable proportions.  Say you don't like eggplant, but you love squash?  You could cut the eggplant and replace it with squash.  Maybe instead of a pound of eggplant and a pound of squash, you have two pounds of squash - one green, one yellow!  Or you love mushrooms, but are meh on bell pepper?  Swap the amounts, and it will still work.  If you make this dish in the fall, you could swap out the summer vegetables for fall vegetables and still make it pretty delicious.

My rule of thumb is, I go by the recipe pretty closely the first time I make it, and I make notes of what I like and don't like.  If my cooking time varies from the recipe, I note that.  If I want to try substitutions, I note that for next time.  If I'm considering substitutions, I think about flavor but I also think about texture and things like cooking time and water content.   Part of the reasons the specific substitutions above work, is because the ingredients are similar.  The mushrooms-peppers swap is the most challenging, but even that is pretty easy to do if you're watching your food as you cook it.  The texture will be slightly different, and you may find that's exactly what you wanted.  Or maybe you'll never make that swap again, but either way you've learned something and given yourself more options in the future.

Is there a recipe in your stock (or family lore) that you consider an elastic recipe?  How do you approach substitutions and stretches?