Heat oven to 400 degrees F. In a medium saucepan, melt butter over medium-high heat and add garlic. Cook, stirring, until just fragrant, about 30 seconds. Stir in flour and slowly whisk in milk. Cook, stirring constantly, until thickened, about 3 minutes. Remove from heat. Stir in crab, 3/4 cup cheese and the juice. Reserve 2 tablespoons scallion and 1 tablespoon jalapeno; stir remaining scallion and jalapeno into crab mixture. Transfer to a 1-quart baking dish and top with remaining 1/2 cup cheese. Bake until heated through, about 20 minutes. Broil 6 inches from heat until golden and bubbly, 6 to 8 minutes. Sprinkle with reserved scallion and jalapeno. Serve warm. Makes 2-1/4 cups.