1. Heat oven to 350 degrees F. Line 2 large rimmed sheet pans with parchment.
2. In food processor, finely chop toasted almonds.
3. In a medium-size bowl, whisk together all-purpose flour, almond flour, baking powder, cream of tartar, salt and nutmeg. In large bowl, whisk eggs to combine. Add vanilla-scented granulated sugar; whisk rapidly for 2 minutes. Blend in oil. Mixture should be lightly thickened. Blend in almond liqueur and almond and vanilla extracts. Stir in ground almonds. Pour flour mixture over egg mixture and stir to form batter-like dough. Mixture will be dense. Let dough stand 2 minutes.
4. With large, sturdy spoon, spoon half the dough onto one of the parchment-lined sheet pans; using a rubber spatula, smooth the length and width into long oval measuring about 5 x 11 inches. Repeat with remaining dough on second cookie sheet. Moisten fingertips with cold water. Lightly press and pat edges of each oval of dough to neaten it.
5. If using the extra slivered almonds, scatter half of them on top of each oval of dough. Press lightly onto top of dough with fingertips.
6. Bake in 350 degrees F oven for about 30 minutes or until golden on top and completely set. A wooden pick inserted in center of each oval must withdraw clean, without any flecks of unbaked dough adhering to it. During baking, dough will expand and rise into larger oval, about 8-1/4 inches at its widest point and about 13-1/2 inches long. It doesnt have to be perfectly formed.
7. Let ovals stand on baking sheets 5 minutes. Carefully transfer them (still on their parchment paper bases) to cooling racks. Let stand for 20 to 30 minutes or until tepid. Ovals must be cool enough to slice evenly and neatly, otherwise they may break or crumble at the edges.
8. Reduce oven temperature to 275 degrees F.
9. Have cooled sheet pans at hand. Working with one oval at a time, transfer to cutting board. With serrated knife, cut each cooled oval into long 3/4-inch thick slices. Arrange rusks, cut side down, on unlined baking sheets.
10. Bake rusks in 275 degrees F oven for 15 to 20 minutes to dry them out or until they are light brown, firm and dry. Turn rusks twice during baking. Let rusks stand on pans 10 minutes. Carefully transfer to cooling racks. Sprinkle cut surfaces with a little sugar, if you wish; let cool completely. Store in airtight tin at room temperature for 4 to 6 weeks.