Heat olive oil in a wide skillet over medium heat. Add chopped onion and cook until softened, about 3–5 minutes.
Add zucchini, yellow squash, garlic, oregano, red pepper flakes, and a pinch of salt. Cook while stirring occasionally for 5–8 minutes until vegetables soften but still have a slight crunch.
Stir in the tomato paste and cook for about 1 minute to deepen the flavor.
Add roasted red peppers, whole peeled tomatoes, and mushroom powder if using. Bring the mixture to a gentle simmer.
Cook the sauce for about 10 minutes until it thickens and reduces. Use a spoon to break up the tomatoes into smaller pieces for a chunky vegetable pasta sauce.
Meanwhile, bring a large pot of salted water to a boil and cook spaghetti according to package instructions.
Remove the sauce from heat and stir in the spinach and fresh basil.
Add the cooked pasta to the skillet and toss everything together. Let it sit for a minute so the pasta absorbs the sauce.
Toss again and serve the Easy Veggie Spaghetti topped with parmesan cheese or nutritional yeast.