Melt the butter in a large saucepan over medium heat. Add onions and cook until tender, about 5 minutes. Season with salt and pepper and add the mushrooms, increasing the heat to medium-high. Cook, stirring occasionally, until almost all liquid has evaporated, about 15 minutes.

Make a sachet from the bay leaf and peppercorns.

Add heavy cream, sachet, and dried thyme, and bring to a boil. Reduce heat and simmer until reduced by about one-third, 35 minutes.

When sauce is ready, discard sachet, transfer cream to a blender and blend smooth, return to saucepan and keep warm. Stir in crème fraîche.

Preheat oven to 200°F.

While the sauce is simmering, melt butter in the oil in a large skillet over medium heat. When melted, add sliced mushrooms, sprinkle with salt and pepper, and fry until golden. Transfer to a plate lined with paper towels to drain. Place in oven to keep warm.

Add portobello slices to the skillet, season with salt and pepper, and fry, turning as needed, until golden. Transfer to a plate lined with paper towels to drain. Place in oven to keep warm.

Bring a large kettle of salted water to a boil and cook pasta according to directions, or until tender, if using homemade pasta. Reserve 1 cup pasta water, then drain thoroughly and toss with butter.

Add pasta to the sauce along with 3/4 of the sliced mushrooms and stir to coat. If the sauce is too thick, thin by adding some pasta water and stirring to combine.

Divide among warmed bowls, topping with remaining mushrooms and portobello slices.

Garnish with flat-leaved parsley and salt.