1. In a medium saucepan, combine the frozen berries with the sugar and 1/2 cup of water and bring to a boil. Simmer over moderately low heat, stirring and crushing occasionally, until the berries and sauce are slightly thickened, about 10 minutes.
2. Meanwhile, in a large baking dish, beat the eggs with the milk. Arrange the bread slices on a work surface and spread 4 of the slices evenly with the almond butter. Sprinkle the 1/4 cup of chopped almonds on top. Spread the raspberry preserves evenly on the remaining slices of bread and close the sandwiches.
3. Heat a large griddle and add 1 tablespoon of the butter. Dip the sandwiches in the egg mixture, pressing to soak. Cook on the griddle over moderate heat until browned, about 3 minutes. Add the remaining butter, flip the sandwiches and cook until golden, about 3 minutes longer. Transfer the French toast to plates and spoon the mixed-berry sauce on top. Garnish with chopped almonds and serve right away.
Make ahead: The mixed-berry sauce can be refrigerated for up to 1 week.