Potato Leek pie (easy! versatile!)
This is a great recipe to use for improvisation, because I think you could sub in and sub out just about every ingredient except the potatoes, milk, cheese, eggs and the puff pastry (and you could even make your own crust, but why?).

1 sheet of puff pastry, the kind from the freezer section, thawed. My store sells them in boxes of 2 sheets. Save the other in the freezer for later.

Thawing one  takes about 30-40 minutes, which gives you time to do all the other stuff.

• 4 medium sized potatoes -- I used Yukon Gold.  
• 3 small leeks or 2 bigger ones
• 1/2 cup (or more) of chopped ham
• 1 bunch of watercress
• 1/2 cup grated or 3/4 "shaved" cheese (I used a mix of romano, asiago and parmesan that my store sells as "salad blend" and which I tend to have on hand.
• 1/3 cup light cream (original recipe called for heavy, I'm sure that's good, too)
• 2 eggs
• salt, pepper, and OK, a tiny bit of cayenne pepper because that's how I roll

Read the instructions for thawing the puff pastry. Thaw one sheet of it, still folded up, at room temp, preferably on a towel or paper towel.
Fill a medium saucepan about half full of water and add a teaspoon of salt. Fire up the burner.
Peel the potatoes and slice them fairly thin. If you have a mandolin or V-chopper, this is a fun time to use it. Otherwise use a knife. 
By now the water should be boiling. Put the potatoes in and cook for about 5 minutes, just until tender or slightly underdone. They get to cook some more later. Drain them so they don't sit in that water and get all soggy.
Clean the leeks and slice them. Just use the white part. Remember to cut them in half first and rinse out the middle. 
Heat some butter or oil in a skillet and cook the leeks just until soft. You don't want them turning golden or brown or black or anything like that. 
Set the oven to 400.

When the puff pastry is thawed, you can open up the folds without breaking it. You may want to put a little flour on your counter or board. Roll it out so it is big enough to line the 9-inch or so pie pan.
Put the pastry in the pie pan and use a fork to prick a few times in the bottom to let the steam out so that part won't puff up too much. Don't prick around the top, though.
Now put in the potatoes, the leeks, the water cress, the ham.
Beat the eggs, add the cream and the cheese, and pour over the other stuff.
When the oven has reached 400, put the pie in. Bake for about 20 minutes until the puff pastry is golden brown.

Obvious substitutions. If you don't have watercress, parsley. Or baby spinach? Arugula! Or any vegetable, although you might pre-cook something like green beans or peas, in with those leeks.
Don't have leeks? Onions? Might be pretty onion-y, so go lighter on those.  Or use a combo of shallots and chives.
No ham? Bacon? Cooked chicken? Leftover turkey? or leave it out and be vegetarian (although obviously not vegan because of the dairy and eggs).
I'm pretty sure any kind of cheese would work.
I'm pretty sure you could add just about anything in moderate amounts. Maybe saute some mushrooms. I would definitely pre-cook most ingredients except the very tender greens.
No cream in the fridge? I'm sure you could use milk. Wouldn't be as rich, but it would work.