This is not a lasagne strictly speaking, but it's quick and easy to knock up for a single portion as a vegetarian. You can use just field mushrooms or a mixture for this.
1-2 sheets of lasagne
A good knob of butter
1 large shallot, peeled, halved and finely chopped
1 clove of garlic, peeled and crushed
150-200g field mushrooms, sliced
120-150ml double cream
1tbsp freshly grated Parmesan
tbsp chopped parsley
Salt and freshly ground black pepper
Melt the butter in a saucepan and gently cook the shallots and garlic for a minute, add the mushrooms and cook with a lid on for a couple of minutes, stirring occasionally until they begin to soften.
Add the cream, season and simmer for a few minutes until the cream thickens. Stir in the Parmesan and parsley; re-season if necessary. There should be a good amount of sauce to coat the mushrooms; if not, add a little more cream and simmer again.
Meanwhile, cook the lasagne sheets in boiling salted water according to the cooking instructions; drain.
To serve, lay a sheet of lasagne on a warmed serving plate, spoon over the mushrooms, reserving a spoonful of the sauce. If you wish, you can lay a second layer of lasagne and spoon the rest of the sauce on top.