In a large frying pan, sauté diced onion in a little oil until golden. Add tomatoes, spinach, and basil. Stir until tomatoes begin to soften. Turn off heat. Add remaining ingredients aside from puff pastry; mix gently to combine. Drain excess liquid from pan.

Preheat oven to 375°F (190°C).

Grease 10 muffin tin cups. Press dough into tins; divide filling between muffin cups. Alternatively, you can skip the muffin tins and do this on a lined baking sheet. After filling dough squares, pinch together opposite corners of dough toward the center, forming a square tart.

Bake 45–50 minutes or until dough is lightly browned and puffed.