Slice the mushrooms into approximately 1/4-inch pieces. I like to keep some of the smaller, more "picturesque" mushrooms whole, to place on top of the dip at the end.
Tip: If you're using a small skillet, you can do the mushrooms in two batches to avoid crowding. The goal here is to get them nicely browned, but if there are too many in the pan, they'll simply steam.
Place a skillet over medium heat, and wait until it's very hot. Add 2 tbsps of the butter (it should start to bubble and foam immediately), followed by the mushrooms, stirring to coat. Continue to stir the mushrooms periodically, until they start to brown nicely. Season with salt and pepper, then add the sage and continue to sauté until the sage has softened and darkened in colour (about 30 seconds to a minute). Transfer to a plate or cutting board, and set aside.
Place a saucepan over medium heat. Once it's very hot, add the butter. When the butter is melted and foaming, add the flour and immediately start to vigorously whisk. Continue to whisk for about 2-3 minutes, or until the mixture has darkened. This is important so the dip does not taste "floury".
Remove the saucepan from the heat as you begin adding the cream and whisk vigorously to ensure no clumping. It will start thickening very quickly. Return it to the heat as you continue to whisk in the cream, followed by the stock. If the mixture is not yet thick, allow it to come to a boil, then lower it to a simmer, continuing to whisk so the bottom doesn't burn. Add the cheese, stirring until it's melted, followed by the mushrooms (reserving some for garnish if you like).
Season the dip to taste, then pour it into a small serving dish and garnish with reserved mushrooms, and extra sage (optional). Serve with 3-4 slices of quality sourdough, halved, toasted, and rubbed lightly with whole raw garlic.
Enjoy!