Heat the oil in a large pot over medium-high heat. Add the beef and ½ teaspoon of the salt and cook until lightly browned, stirring frequently while breaking up the meat into bite-size pieces, about 3 minutes. Add the garlic, oregano, and red pepper flakes (if using), and sauté for 1 minute.
Add the bell peppers, mushroom, and remaining ½ teaspoon of the salt (if your meat is lean, add another tablespoon of oil). Cook, stirring occasionally, until the mushrooms soften, about 5 minutes.
Add the tomatoes. Bring to boil, reduce heat to a simmer, and cook, uncovered, stirring occasionally, until the sauce thickens, about 25 minutes. Stir in the spinach or kale (if using).
Taste and add more salt and pepper, if needed.
Serve over pasta, Rest Interval (RI)e, zucchini noodles, or spaghetti squash and top with grated Parmesan (if using)