Lightly toast the flour, nuts and oats in a pan, then cool. Combine the flour, sugar, cinnamon, oats and salt. Using a pastry blender, cut in the butter, then bake for 20 minutes at 325 or until lightly browned and toasted. Cool the struesel and reserve. From here the streusel can be made ahead and frozen in a labeled container for future use.
Over a double boiler, whip the egg yolks and sugar until they are light, fluffy, and doubled in size.
Transfer the whipped yolks to a non-reactive bowl and mix in the apple butter and the cream.
Transfer the mixture to 4 ounce ramekins that are lightly oiled or lined with plastic wrap, then freeze overnight until set.
To serve, un-mold the mousses by sliding a paring knife in the ramekin around the mousse to release it, then slide it onto a small bed of struesel on each dinner plate, then garnish with candied apple chips, angelica leaves or stems, or whatever you like.
I like to serve the mousse with something crunchy or crumbly underneath, pictured is a streusel made with black walnuts. This recipe can easily be scaled, and you can serve it with whatever garnishes you want.