Pasta: Cook the tagliatelle according to the package instructions, drain and transfer to an oven-safe casserole dish.
Ratatouille: In a pan heat oil and saute onion and garlic until soft. Add zucchini, bell pepper, aubergine, mushrooms, tomato paste and oreganum. Season to taste with salt and pepper. Gently saute to cook to just tender but not mushy.
Cheese Sauce: Melt the butter or margarine in a saucepan. Add the flour and whisk to form a roux. Add milk and stir continuously until thickened. Remove from the heat and stir in mustard, grated cheese and season with salt and pepper.
Assemble: Spoon the ratatouille over the pasta and top with cheese sauce and bake at 180°C for 15 minutes.