In a large saucepan, place a steamer basket over 1 in. of water. Place cauliflowerets in basket. Bring water to a boil. Reduce heat to maintain a simmer; steam, covered, 10-12 minutes or until tender. Cool slightly.
Place cauliflower in a food processor; process until pureed. Transfer to a large saucepan; stir in tomatoes, spinach, salt and garlic powder. Bring to a boil. Reduce heat; simmer, uncovered, about 2 hours or until flavors are blended, stirring occasionally. Serve with pasta.