Adjust oven rack to middle position and heat oven to 450 degrees. Melt 2 tablespoons butter in large saucepan over medium-high heat. Cook onion until softened, about 5 minutes. Add artichokes and cook until lightly browned, about 5 minutes. Stir in garlic and cook until fragrant, about 30 seconds. Transfer artichoke mixture to plate and reserve.
Melt remaining butter in empty pan. Stir in flour and cook until just golden, about 1 minute. Slowly stir in half-and-half, 11/4 cups Parmesan, lemon juice, hot sauce, and salt. Reduce heat to medium-low and simmer until thickened, about 3 minutes. Off heat, stir in spinach and reserved artichoke mixture.
Transfer to 1-quart soufflÇ or baking dish and sprinkle with remaining cheese. Bake until golden brown and bubbling, about 15 minutes. Cool 5 minutes. Serve.
Make Ahead: Dip can be prepared through step 2 and refrigerated in an airtight container for up to 24 hours. To heat, let dip sit at room temperature for 1 hour, then sprinkle with cheese and bake, covered with foil, in a 450-degree oven for 10 minutes. Remove foil and bake until golden brown and heated through, about 15 minutes longer.