In a large skillet, heat oil over medium-high heat. Add sausage and onion; cook and stir until onion is tender and sausage is browned, 3-5 minutes. Add curry paste and garlic; cook 1 minute longer., Stir in the tomatoes and coconut milk. Bring to a boil; reduce heat. Simmer, uncovered, until sauce starts to thicken, 7-10 minutes. Add spinach, salt and pepper; cook and stir until spinach begins to wilt. Serve with rice.