Gather all ingredients. Preheat the oven to 375 degrees F (190 degrees C).
Heat 1 tablespoon sun-dried tomato oil in a 10-inch enamel-coated cast-iron skillet over medium. Add garlic and tomato paste and cook, stirring constantly, until garlic is fragrant and tomato paste is slightly darker in color, 2 to 3 minutes.
Remove the skillet from heat and whisk in cream cheese and sour cream until smooth. Fold in spinach, Parmesan, Italian seasoning, pepper, salt, sun-dried tomatoes, and 1 cup mozzarella until evenly combined.
Spread tomato mixture into an even layer. Sprinkle evenly with remaining ¼ cup mozzarella cheese.
Bake in preheated oven until cheese is fully melted and edges are bubbling, 8 to 10 minutes. Without removing skillet from oven, increase oven temperature to broil. Broil until cheese is lightly browned on top, 3 to 5 minutes. Garnish with basil and serve immediately with crostini.
