Before starting, preheat the oven to 450˚F. Wash and dry all produce. Unroll pastry, discarding the wax paper, then place on a parchment-lined baking sheet. (NOTE: for 4 ppl, use 2 baking sheets.) Using a small knife, score a 1/2-inch border around pastry. (NOTE: Make sure not to cut all the way through!) Using a fork, poke centre of tart all over, avoiding the border. Bake tart in the middle of the oven, rotating halfway through, until pastry is golden-brown and cooked through, 18-22 min. (NOTE: for 4 ppl, bake in the middle and top of the oven.) While pastry bakes, thinly slice mushrooms. Peel, then cut onion into 1/4-inch pieces. Peel, then mince or grate garlic. Strip thyme leaves from stems, then finely chop. Roughly chop half the spinach. Halve tomatoes.

Heat a large non-stick pan over medium-high heat. When hot, add 2 tbsp butter (dbl for 4 ppl), then onions, garlic, mushrooms, half the thyme and half the vinegar. Season with salt and pepper. Cook, stirring often, until mushrooms have softened and liquid is absorbed, 8-10 min. Sprinkle flour over mushrooms and cook, stirring often, until golden-brown, 1-2 min. Add broth concentrate and 1/3 cup water (dbl for 4 ppl). Cook until filling thickens slightly, 1-2 min.

While mushrooms cook, rip or tear bocconcini into a large bowl. Add remaining thyme, 1/4 tsp salt and 1/2 tbsp oil (dbl both for 4 ppl). Season with pepper, then toss to combine.

When filling is done, add chopped spinach to the pan with filling. Stir until wilted, 1 min. Set broiler to high. Spoon filling over pastry in an even layer, avoiding the border, then top with bocconcini. Broil in the middle of the oven until bocconcini melts, 2-4 min.

While bocconcini melts, add remaining vinegar, 1/2 tsp sugar and 1/2 tbsp oil (dbl both for 4 ppl) to the same large bowl (from step 3). Add tomatoes and remaining spinach. Season with salt and pepper, then toss to combine.

Transfer tart to a clean cutting board to cool, 2-3 min. Cut into quarters. Divide tart and salad between plates.