Make the crust: In a food processor, blend the oats into a coarse powder.
Add the butter to a medium saucepan over medium heat and cook until nutty brown in color, 5-6 minutes. Once the butter has browned, add the ground oats, flour, salt, and water, and mix to combine. Let cool for 5 minutes.
Transfer the crust mixture to a 9-inch (20 cm) tart pan, spreading evenly across the bottom and up the sides. Chill for 30 minutes.
Preheat the oven to 375˚F (190˚C).
Make the filling: Place the apples in a large microwave-safe bowl and squeeze the lemon juice over, then toss to coat. Microwave for 4 minutes. Cover with a plate and let steam for 5 minutes.
Season the apples with the cinnamon and nutmeg, and toss to coat.
Spread the applesauce over the bottom of the crust.
Starting from the outside and working inward, arrange the apples in the pan in a circular pattern with the curved side up. Begin with the larger slices, overlapping slightly, and continue until the entire crust is filled. Brush the apples with the melted butter.
Bake for 1 hour, checking occasionally. If the edges are browning too quickly, cover gently with foil to prevent further browning as the tart cooks through.
If using, microwave the apricot jam for 20-30 seconds to melt. Use a pastry brush to paint the apricot jam over the warm tart for a nice, shiny finish.
Let the tart cool to room temperature before slicing and serving.