Preheat the oven to 400 degrees F, and lightly grease a baking sheet or line it with foil for easy cleanup. Using a sharp knife, very carefully cut the spaghetti squash in half lengthwise. (I find it helpful to microwave the squash for about 3 minutes before cutting to soften it. Pierce it once along the line where you will cut it later so the steam can escape.) Scrape out the seeds with a spoon, and place cut-side up on the prepared baking sheet. Mist or brush with olive oil (I usually drizzle a teaspoon of oil around the edges and inside each half, and then rub it all around with my fingers). Season the flesh well with salt and pepper. Flip the squash and bake cut-side down for 25-35 minutes or until the squash meets little resistance when pierced with a knife and the flesh easily pulls away from the shell. (Exact time will depend on oven, size of squash, and if you microwaved to soften.) Using a fork, shred the inside of the squash until the flesh is spaghetti-like, leaving about 1/4-inch of the squash in tact around the border so the shell doesn’t collapse.
Add approximately 1/2 cup of the pizza sauce and 1/4 cup of shredded cheese to both squash halves. (If using sautéed mushrooms or cooked sausage as toppings, I like to stir some of them in with the squash, too.) Toss to evenly coat the squash with the sauce. Top each half with 1/4 cup additional cheese and a few pepperoni slices or toppings of choice.
Turn the oven to broil, and with a rack about 1/3 from the top of the oven, broil for 2-3 minutes, watching very carefully so they don’t burn, or until the cheese is completely melted.