Heat the oven to 450°F with the rack in the middle. While the oven is getting hot, clean your cremini mushrooms and remove the stems.
Place the mushrooms on a foil- or parchment-lined baking sheet, and brush them with melted ghee. Roast the mushrooms, gill-side down, for 12 minutes.
Flip over the mushrooms, and roast for 5 to 10 minutes more, or until the liquid released by the mushrooms has evaporated.
While the mushrooms are in the oven, de-vein and remove the tails from the thawed shrimp, and chop them into medium-sized pieces. Toss the shrimp, scallions, cilantro, bacon, jalapeño, and fish sauce into the work bowl of a food processor.
Pulse the ingredients for the filling until a coarsely-chopped mixture develops. The filling should be the consistency of a sticky, chunky paste. To taste for seasoning, form a tiny patty and fry it up and eat it. Add extra salt and pepper to the stuffing if needed.
Scoop out the filling with a spoon or small disher, and fill each cooked mushroom with the shrimp paste.
Return the mushrooms to the oven for 8 to 10 minutes, or until the shrimp mixture has set. Transfer the mushrooms to a plate and serve immediately.