Peel the potatoes, cut into similar size pieces (halves or quarters) and boil in salted water with the bay leaf until tender but take care not to overcook them. Place in a medium casserole dish and set aside.
Preheat the oven to 400 F/ 200 C/ gas mark 6.
Heat up a large non-stick pan and fry the mushrooms for 3-4 minutes over a high heat until lightly browned, stirring once or twice. Scatter the mushrooms along with the chopped asparagus over the potatoes. Season.
To make the herb sauce, in a saucepan melt the butter over a low heat, add the flour and whisk until the mixture is smooth. Add some of the milk, increase the heat and continue whisking until the mixture has thickened. Add the rest of the milk and continue whisking until the sauce starts bubbling up. Add the fromage frais and remove from the heat. Stir in the herbs, season to taste and pour over the casserole.
Sprinkle the grated cheddar over the whole thing, drizzle with a little olive oil and bake for 20-25 minutes. Remove from the oven and serve immediately.
The asparagus tips shouldn't be too thin or too thick so go for something in the middle. This is to ensure that after 25 minutes in the oven they are cooked but still crunchy (very thin asparagus might be too tender and very thick tips might still be tough).
As the potatoes don't get mashed in this casserole I recommend choosing potatoes that hold their shape well in cooking. Such as salad potatoes. Waxy potatoes, such as Charlotte or Anya are good options, while Maris Piper, Marabel or Rooster work better in mashed potato dishes. If you live over the pond Yukon Gold would be a good choice (Russet or Idaho wouldn't).
I used chives and parsley but you could just go with one or the other, or omit herbs altogether.