For this make-ahead dessert, assemble everything, including the blueberry and marshmallow checkerboard design. Refrigerate up to a day ahead. At serving time, broil the tart briefly to brown the marshmallows.
Heat oven to 375°F.
Place graham crackers in large plastic food-storage bag; seal. Crush with rolling pin until coarsely crumbed. Place almonds in food processor; process until finely ground. Add sugar, melted butter and crumbs; pulse until well combined and crust begins to stick together.
Press crumb mixture over bottom and up sides of 11 x 7 x 1-1/2-inch rectangular baking pan.
Bake in 375°F oven for 8 to 10 minutes or just until crust begins to brown around edges. Remove pan to wire rack and let cool.
Combine 2 cups blueberries, the sugar, cornstarch, water, lemon juice, nutmeg and almond extract in medium-size saucepan. Cook over medium-high heat, stirring, until mixture is clear, blueberries pop and mixture thickens, about 10 minutes.
Pour blueberry filling into baked crust, spreading to edges. Arrange remaining 1 cup berries and the mini marshmallows in an alternating checkerboard pattern over top of tart. Cover with plastic wrap and refrigerate until ready to serve.
Remove tart from refrigerator 30 minutes before serving. Heat broiler.
Watching carefully, place tart under broiler just until marshmallows begin to brown, 1 to 2 minutes. Remove from oven. Remove side of pan and serve immediately. Make-Ahead: Crust can be made ahead and stored at room temperature for up to 1 week. The tart can be assembled and refrigerated a day ahead.