Heat 2 Tbsp. olive oil in a large skillet with a fitted lid over medium heat.
Add diced onion and halved cherry tomatoes; cook 7 to 8 minutes until onion is soft and tomatoes break down.
Add penne pasta, marinara sauce, water, garlic powder, dried oregano, salt, and pepper.
Increase heat to medium-high, bring to a simmer, then cover and cook, stirring occasionally, for 10 to 12 minutes until pasta is al dente.
Remove lid and stir in heavy cream, fresh spinach, and shredded mozzarella cheese.
Cook while stirring continuously for about 2 minutes until spinach wilts and cheese melts.
Garnish with chopped fresh basil and grated Parmesan cheese if desired.
Serve warm and enjoy your creamy tomato basil pasta dinner.