Heat oven to 200C/fan 180C/gas 6. Deseed the peppers, halve, then cut into large chunks. Trim ends off zucchini and eggplant, then cut into slices about ½cm thick. Lightly grease 2 large baking trays, then place peppers, zucchini and eggplant on top. Toss with the olive oil, season well, then roast for 25 mins until lightly browned.
Reduce oven to 180C/fan 160C/gas 4. Lightly oil an ovenproof serving dish (30 x 20cm). Arrange a layer of the vegetables on the bottom, top them with some baby spinach leaves then pour over a third of the tomato sauce. Top with a layer of gluten free lasagne, then drizzle over a quarter of the white sauce. Repeat until you have 3 layers of pasta.
To finish, spoon remaining white sauce over the pasta, making sure the whole surface is covered. Scatter mozzarella over the top with the tomatoes. Bake for 45 mins until bubbling and golden.