Tomato Tart with Seared Tuna and Caramelized Onions

This is based on Cal Stamenov's recipe which is in the recipe booklet that I received when I attended the 1998 TomatoFest at Quail Lodge in Carmel, California.

1 lb. puff pastry (Dufour is a good brand, it's 14 ounces which is enough)
2 medium onions, sliced thin - 1/16 inch crosswise
2 tbsp olive oil
1 branch of fresh thyme
1 tbsp sugar (or 1 tsp if using sweet onions)
2 cloves garlic, finely chopped
1 tsp soy sauce
1 1/2 tbsp vinegar (rice vinegar works well)
1 lb. Ahi Tuna (I've also used local Albacore tuna), sliced 1/2 inch thickness
fresh lemon or lime juice
1 tbs vegetable oil
3 medium tomatoes, sliced
3 tbsp fresh basil, chopped

Vinaigrette:
1/4 cup olive oil
1 clove garlic, finely chopped
3 tbsp Balsamic vinegar
salt and pepper

Preheat the oven to 400ºF. Roll the cold puff pastry into a rectangle onto a floured surface, approximately 1/8-inch thick. Trim the edges off and cut into four equal portions (squares or squarish). Place the portions on a baking sheet. Prick each piece 12 times with a fork and bake for 15 to 20 minutes, until golden brown. Remove from the oven and keep them on the baking sheet.

Sauté the onions with oil, thyme, and sugar for 20 to 25 minutes, until light brown. Deglaze with soy sauce and vinegar. Add the garlic and cook for another 5 minutes.

Season the tuna slices with salt, pepper, and lemon or lime juice. Heat a skillet over medium-high heat, swirl in the vegetable oil and sear the tuna on both sides for 20 seconds. Remove from the pan.

Assembly: Spread the onions evenly over the cooked puff pastry squares (do not remove from the baking pan). Slice the cooked but rare tuna diagonally into 1/4-inch thickness. Arrange the tuna evenly over the onions. Arrange the tomato slices over the tuna and warm them under the broiler for 2 to 4 minutes. Sprinkle with the basil and drizzle with the garlic vinaigrette. Serve immediately.

Serves 4 as a main course, or slice into smaller portions and serve as an appetizer for more.