Heat the oven to 375°F and arrange a rack in the middle.Heat butter in a medium saucepan over medium heat. When it foams, add onion and garlic, season with salt and freshly ground black pepper, and cook until onion and garlic are beginning to turn brown in color, about 6 minutes. Stir in flour and cook stirring constantly until no longer raw tasting, about 1 minute.Sprinkle in 3/4 cup of the Parmesan, pour in cream, and stir to incorporate. Add artichoke hearts, sour cream, and 1/2 cup of the cheese and stir until cheese begins to melt.Transfer dip to an 8-by-8-inch baking dish and top with remaining Parmesan and Gruyère and the panko. Bake until heated through, bubbling on the edges, and the cheese is melted, about 30 to 35 minutes. Let rest 5 minutes. Serve with crackers or pita chips.