In a medium skillet over low heat, heat the olive oil and butter and cook the onions and garlic until caramelized and golden, 20-30 minutes. Transfer to a plate, season with salt and pepper and set aside. Over medium heat, bring honey to a gentle simmer, swirling the skillet but do not stir, until warmed through and slightly thickened, 5-6 minutes. And vinegar and mustard, swirl pan to combine, 2-3 minutes. Add tomatoes, thyme, salt and pepper and cook for 5 minutes. Let tomatoes cool completely.
Preheat over to 375°. Lightly flour work surface as roll out puff pastry until 1/8” thick. Cut circles of dough slightly larger than tart pans and transfer to a very lightly floured sheet pan and refrigerate until ready to assemble.
Lightly butter each individual tart pan, make a single layer of tomatoes, skin side down. Put a layer of caramelized onions divided evenly over the tomatoes. Place a round of puff pastry directly over the onions, and brush with a lightly beaten egg. Sprinkle lightly with salt. Bake until pastry is puffed and deep golden brown, 15-20 minutes.
Let cool for 10 minutes, then run a small knife around the edges to loosed if necessary and invert onto a plate. Garnish with a dollop of ricotta and a fresh sprig of thyme.