Preheat the oven to 190°C (fan 170°C/gas 5). Cut vegetables into 1.5cm discs, toss with olive oil and garlic, and spread on a baking tray. Roast for 30-35 minutes until tender and slightly golden.
Increase the oven temperature to 200°C (fan 180°C/gas 6).
Melt butter and maple syrup in a pan, then pour into the bottom of a 26x36cm baking tray. Add rosemary and arrange the roasted vegetables in a single layer.
Roll out pastry, join the pieces with egg, and drape over the vegetables. Trim, tuck in the sides, and make steam holes. Glaze with egg and bake for 30-35 minutes until golden and puffed.
Let the tart cool for 5 minutes, then flip onto a board and cut into sections.
For the cheese sauce, melt the ingredients in a pan, season with white pepper, and serve with the tart.