Here is why this is a bits and pieces recipe. I had only about 1.5 cups of gluten free pasta, about 20 pepperoni left from a pizza recipe and the ingredients to mix up some spaghetti sauce. Lacking any mozzarella cheese, I improvised with some white American instead. The results.............amazing.

Spaghetti Sauce
In 2 tablespoons olive oil, heat 1/2 chopped onions, 2 wedges of garlic and 1/2 chopped mushrooms if desired. Stir well while heating. While that is simmering, add the package of seasoning, 1 cup water and the can of tomato paste to a mason jar. Add an air tight lid and give it a thorough shake. Add to the sauteed veggies and stir on low. At this point, I add salt, pepper and basil. Leave on low. The secret to great sauce is to add liquid from cooking pasta. Seriously. I add 1 - 2 cups of pasta liquid to my sauce. You can add meat if you want, but it increases the cost.
I have enough sauce for 2 meals, so 1/2 of what is simmering goes back into the mason jar, sealed and stored in the refrigerator until next time.
Back to the Pasta Bake
In a casserole dish, spread a small amount of sauce. Layer drained, cooked pasta over the sauce. Then, layer the pepperoni over the cooked pasta. Add some shredded cheese, about 1/2 cup only. You can add more cheese if you want. Then, sprinkle 1 cup of sauce over the the pasta / pepperoni mixture. Cover with plastic and cook in the microwave for 8 minutes; or, cover with foil and cook in the oven for about 20 minutes. The pepperoni should start to shrink and curl.
Once cooked, remove from oven or microwave and let cool slightly. At this step, you can also sprinkle more shredded cheese over the mixture and cook just a bit longer, if desired. Remove the covering and serve. You can sprinkle some Parmesan cheese over it, if you want.
Leftover Version
If you are using a leftover pasta, you just need to add a couple of ingredients and warm through. You can use any leftover marinara based pasta: lasagna, spaghtetti, ziti. If you want to experiment with alfredo based pastas, the results may not be as good as you want. I do something different with these.
There are many other leftover recipes on the internet. As I researched them, I added a couple to my "try this later" list, especially the zucchini spaghetti pancakes. The main idea is to use leftovers and to remain frugal.