Stocking a pantry efficiently is a different question than what to do to use up fresh vegetables before they rot, which seems to be your main concern.
You can make that lettuce up into salad all at once that will keep in the fridge for the few days it takes to eat it. I keep mine in my salad spinner wrapped in a paper towel to absorb any excess moisture. It's not a bad habit to have to eat salad with your dinner. Take out and dress what you need each night. Keep a basket of those little grape tomatoes on hand to slice into the bowl. Almost any other raw-and many cooked--vegetables you have hanging around can be added to a salad to make it interesting. So can bits of fresh or dried fruits and nuts or seeds. Chop a little bit of those carrots and that celery into it, while you're at it. Or cut the carrots and celery into sticks so they are ready for you to take them to lunch with you. I know it's boring, but they won't go to waste.
Celery and especially carrots keep for quite awhile and are the basis, with onions, of a lot of good things to eat. Heck! Trader Joe's charges a good bit for 1 lb containers of pre-chopped mirepoix. You can use those three things as the basis for almost any kind of soup. If those veggies are really getting on, turn them into vegetable stock to freeze for later use. Almost any aging vegetable you have on hand can be added to the pot for stock. You could add that half-onion. Or add them to a chicken carcass or two to make chicken stock. (I freeze my roasted chicken carcasses until I have 2 or 3 saved to make stock.) As for that onion, I probably would have just ignored the recipe and put the whole thing into the stew.
How to stock your pantry beyond the very basics, like sugar, flour, pasta, rice, oils and vinegars will depend a lot on what you like to cook and eat.