Roasted tomato tart with double cheese crust

Name: Roasted tomato tart with double-cheese crust
Ingredients: 3-4 onions, sliced; 1 tbsp butter; olive oil, for drizzling; 2 tsp sugar, plus a pinch; 2 thyme sprigs leaves stripped, plus an extra sprig; 5 sage leaves, shredded then finely chopped; 85g fresh breadcrumb; 25g fresh parmesan (or vegetarian alternative), grated; 500g tomato (a mix of varieties looks great), thickly sliced; 300g plain flour, plus extra for dusting; 100g cold butter, cubed; 85g mature cheddar, cubed; ½ tsp salt; 25g parmesan (or vegetarian alternative), grated; green salad, to serve (optional)