Heat the broiler to high (500°F) with a rack 6 inches from the heat. Heat the olive oil in a 12-inch oven-safe skillet over medium heat. Add the garlic and sun-dried tomatoes; cook, stirring, 1-2 minutes until fragrant.
Add the beans, broth, Italian seasoning, 1/4 tsp salt and a few grinds of pepper. Simmer 10 minutes, stirring occasionally.
Mash about one-third of the beans against the skillet with the back of a spoon. Stir in the milk and 1/2 cup Parmesan; simmer 2-3 minutes until lightly thickened.
Add the spinach in handfuls, stirring until wilted, about 2 minutes. Stir in the lemon juice.
Sprinkle the remaining 1/4 cup Parmesan over the skillet.
Broil 2-3 minutes until the top is golden and bubbling. Serve hot.