Bring a large pot of water to a boil. Add the spaghetti and cook according to package directions. Drain the pasta into a colander and return it to the pot. Toss with a few dashes of olive oil.
While the spaghetti cooks, coat the bottom of a large saucepan with 2 tablespoons of olive oil and place it over medium heat. When the oil is hot, add the mushrooms in as even a layer as possible, and allow them to cook until they're browned on the bottom, about 5 minutes. Flip and cook 5 minutes more, until they're browned on the other side. Remove them from the pan and transfer them to a plate.
Add the remaining 2 tablespoons of olive oil to the saucepan. Once hot, add the bell pepper, garlic and red pepper flakes (or fresh hot peppers). Sauté until the bell peppers are tender, about 5 minutes, watching closely and flipping frequently to avoid the garlic burning. Return the mushrooms to the pot and stir in the diced tomatoes, tomato paste and sugar. Raise the heat and bring to a simmer, then lower heat and simmer for 10 minutes, until the liquid thickens up a bit. Remove from heat and season with salt and pepper to taste. Stir in the fresh basil. Optionally, add a few extra red pepper flakes for more heat.
Pour the sauce over the cooked spaghetti and toss to coat. Divide the pasta onto plates and sprinkle with fresh basil and some additional red pepper flakes, if desired, and serve.
Fresno or cherry peppers are both great choices if you choose to use fresh hot peppers for this recipe, but most common varieties will work just as well. Just keep in mind that some are hotter than others, and adjust the amounts as needed. Feel free to swap out the mushrooms and bell pepper with your favorite veggies. Hard veggies like cauliflower and carrots can be added to the pot along with the tomatoes, while softer veggies like zucchini can go in about halfway through the simmer time. You'll need about 2 to 3 cups of whatever you use.