With the flavors of balsamic roasted shallots coated in a delicate whole-grain mustard and thyme sauce, this tart is easy to make and looks stunning enough to be the centerpiece of a dinner party. I like to use banana shallots as they are larger than the other shallots, so quicker to peel and prepare. But if you cannot find them, then the same weight of normal shallots works perfectly well too (in which case they would not need to be cut into quarters).
Lay the prepared shallots on a baking sheet, drizzle over 1 tablespoon of the olive oil and the balsamic vinegar, then sprinkle over 1 tablespoon of the whole thyme leaves and the sea salt. Gently mix until the shallots are well coated. Cover with foil and roast for 20 minutes, then uncover, toss gently, and cook for a further 10 minutes, until the edges of the shallots start to turn golden brown. (Maybe a little more or less, depending on the size of shallots as they can vary in size).
Place a clean baking sheet in the oven to heat up. This will allow the base of the pastry to cook and become crisp underneath.
To make the sauce, add the remaining 1 tablespoon olive oil and the 1 tablespoon chopped thyme leaves to a medium pan over medium heat and let it heat up.
Meanwhile, to a measuring cup, add the plant-based milk, cornstarch, vegetable bouillon powder and whole-grain mustard and whisk with a fork. Then gradually pour into the saucepan, stirring constantly with a wooden spoon. Add the white wine and allow the sauce to gently bubble and simmer to cook off the wine and allow the sauce to thicken so it coats the back of the wooden spoon, 4 to 5 minutes. Turn off the heat.
Take the shallots out of the oven and turn the temperature up to 400 degrees F.
Gently fold the roasted shallots into the sauce, taking care they hold their shape.
Unroll the puff pastry and roll out so it fits into a rectangular 11-by-8-inch fluted tart tin with a removable bottom (if your pastry came as 2 sheets, press them together to create 1 large sheet). Place the pastry in the tin and gently push into the corners. Prick the base of the pastry several times with a fork.
Sprinkle over the cooked lentils then gently arrange the shallots on top in a vertical pattern for presentation. Drizzle over any remaining sauce.
Place the tart tin on the heated baking sheet in the oven and bake until the pastry is cooked through and golden brown, about 40 minutes.
Remove the tart from the oven, gently push up the base to remove the tart from the tin and slide onto a serving platter or wooden board. Garnish with the final tablespoon of whole thyme leaves. Slice into 4 or 6 pieces, depending on preferred serving size.