Toss the apples with the lemon juice and set aside. Melt the butter and brown sugar over medium heat, stir in 1/4 c. cream and bring the mixture to the boiling point, stirring constantly for 1-1/2 minutes. Remove from heat and whisk in the remaining 2 Tbsp. of cream.
Stir the egg with a teaspoon of water and brush the mixture over the bottom of the tart pan, place the apples evenly and decoratively over the bottom of the pan, and drizzle the caramel over the apples. Fold the dough over the edge of the apples and brush any exposed surface with the egg wash.
Place the tart pan in the oven, directly on the rack. You may wish to place a cookie sheet with parchment or aluminum foil below it, in case any of the filling bubbles over. Bake for 12 minutes and reduce the temperature to 350 degrees F. without opening the oven door. Bake for another 30 to 40 minutes, until the filling is bubbly in the center and the crust is golden brown.
Remove and cool for about 20 minutes before serving.