For the filling, crisp the bacon in a small skillet (add 1 tablespoon avocado oil if using turkey bacon), and then remove it, let the bacon cool, and then crumble.
In the same skillet that the bacon cooked in, cook the onion over medium heat until softened, but not browned, about 5 minutes, stirring occasionally. Cool.
Whisk together the cream, egg, egg yolk, salt, black pepper, and nutmeg in a medium bowl and set aside.
For the crust, stir together the almond flour and salt in a large bowl, and then cut in the butter with a fork. Mix in the egg white until it forms a dough. (The dough will be crumbly, but will hold together when squeezed.)
Press the dough into the bottom and about ½-inch up the sides of a 9-inch tart pan. Sprinkle the bacon in the bottom, and then the cheese. Pour the egg mixture on top, and sprinkle on the scallion.
Bake until the eggs are set and the crust is golden, about 35 to 45 minutes.