For the Crust
Preheat oven to 375 degrees F. Lightly oil a 9-inch tart pan with a removable bottom or coat it with nonstick spray. Spread oats in a pie pan; bake for 7 to 15 minutes, or until toasted. Let cool. Process in a food processor until finely ground. In a large bowl, stir together ground oats, flour, sugar, baking powder and salt. Drizzle oil onto dry ingredients and mix with a fork or your fingers until crumbly. Use a fork to stir in water, 1 Tbsp. at a time, until dough just comes together. Turn dough out onto a floured surface; knead 7 to 8 times. Roll dough out into an 11-inch circle. Transfer dough to prepared pan, pressing to fit. Trim edges. Prick bottom of crust in a few places with a fork and line with foil or parchment paper. Fill with pie weights, dry beans or rice.
Bake crust for 8 to 12 minutes, or until firm. Remove foil or parchment and weights; cool on a wire rack.
For the Filling
In a bowl, mix 3 Tbsp. sugar, lemon juice, lemon zest and cinnamon. Peel, core and cut apples lengthwise into 12 wedges each, tossing apples in sugar mixture as you work. Arrange apples in a single layer in prebaked crust and drizzle with any liquid from bowl. Place pan on a baking sheet and bake for 20 to 25 minutes, or until apples are tender but still hold their shape. Meanwhile, in a food processor, puree ricotta cheese until smooth. Add cream cheese, egg, vanilla, salt and remaining 3 Tbsp. sugar; process until smooth. Pour over apples.
Bake tart for 15 to 20 minutes more, or until custard is set. Let cool on a wire rack for 20 to 25 minutes. Serve warm or chilled.