Adjust the oven rack to the middle position and heat the oven to 450°F.
Melt 2 tablespoons butter in a large saucepan over medium-high heat. Saute the onion until softened, about 5 minutes. Add the artichokes and cook until lightly browned, about 5 minutes. Stir in the garlic and cook until fragrant, about 30 seconds. Transfer the artichoke mixture to a bowl and set aside.
Melt the remaining butter in an empty pan. Stir in the flour and cook until just golden, about 1 minute. Slowly stir in the half & half, 1¼ cups Parmesan, lemon juice, hot sauce and salt. Reduce the heat to medium-low and simmer until thickened, about 3 minutes. Remove from heat and stir in the spinach and artichoke mixture.
Transfer to a greased 1-quart baking dish and sprinkle with the remaining ¼ cup cheese. Bake until golden brown and bubbling, about 15 minutes. Cool 5 minutes before serving.
Serve warm with crackers or sliced baguette.
Notes
To make ahead: Prepare the whole dip (but don't bake it). Cover tightly for up to 24 hours. To heat, let the 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