Heat olive oil in a large skillet over medium heat. Add onions and sauté for 3-4 minutes until softened. Add bell peppers, garlic, and red pepper flakes (if using). Cook 8-10 minutes, stirring occasionally, until peppers are tender and slightly caramelized. Stir in cherry tomatoes and continue cooking 2-3 minutes until tomatoes soften. Season with salt, black pepper, and lemon juice. Cook spaghetti according to package instructions. Toss cooked pasta with the pepper mixture until evenly coated. Add a splash of pasta water to loosen if needed. Top with Parmesan, butter (if using), and fresh parsley before serving.