Cut butter into small cubes. Combine flours, salt and baking powder. 

Using a pastry blender, cut butter into the flour mixture until the largest pieces are the size of peas. 

Stir in water until dough holds together. Pat dough into a disk and wrap tightly in plastic. Chill until firm, at least 2 hours.

Sauté onion in olive oil, season with salt and pepper. Set aside to cool.

Unwrap dough and roll out, on a lightly floured surface, to roughly 1/8" thick.

Gently roll dough onto rolling pin and lay inside of tart pan. Press down bottom and sides, making sure to press into every groove.

Roll pin over top of tart pan to evenly "cut" dough across the top.

With a fork, poke holes in bottom and cover dough with a piece of foil or parchment paper.

Place beans or pie weights inside and bake at 350 degrees for 15 - 20 minutes. Remove foil and put back in the oven for another 10 minutes or until light golden brown.

Place onions in the bottom of tart in an even layer, sprinkle goat cheese on top of onions.

Shingle tomato slices around edge of tart, place grape tomatoes in the middle, cut side up. Season again with salt and pepper.

Sprinkle Parmesan cheese and thyme leaves on top.

Bake at 400 degrees until cheese is burnished and bubbly, about 20 minutes, rotating halfway through if necessary.