Prep & coarsely chop veggies:
2 stalks celery
3 cloves garlic
1/4 C parsley
Optional: other veggie scraps
Combine the above in pressure cooker with 2-4 C water (depending on the size of your pressure cooker- following instructions for how far you can fill the pot). Add spices:
1-2 sprigs rosemary
1/4 - 1/2 tea whole peppercorns
1-3 bay leaves
Optional: other spices or herbs
Put on lid, bring to pressure and cook, pressurized over low heat, for 20-25 minutes. Then let pan come down in pressure naturally ("slow release" method). Strain and use in soups, mashed potatoes or wherever else you'd use stock.
Freezes well- just put in chunks that you will actually use.
For more directions or ideas, see this NY times article on the same topic.