Heat a skillet large enough to hold the mushrooms in a single layer over medium-high heat. Add 1 tablespoon of butter, which should, ideally, melt quickly and turn a light brown color. Add the shallots, season with salt and pepper, and give them a quick stir. Add mushrooms, 1 more tablespoon of butter and season with a bit more salt. Cook for about 4 minutes until mushrooms begin to turn golden brown and release some of their water. To avoid a possible flame up, move pan away from the stove and carefully add the vermouth. Return pan to stove and let cook for 2 to 3 minutes to reduce the vermouth.
While mushrooms are cooking melt remaining 1 tablespoon butter in a small skillet over low heat. Add the hazelnuts and let them "simmer" in the butter over low heat until they turn light brown, about 4 minutes.
Once the vermouth has reduced and been absorbed by the mushrooms sprinkle in the paprika (use a mesh strainer to evenly distribute the paprika over all the mushrooms). Add the lemon juice and hazelnuts and give it a stir. Remove from heat and let cool for 2 minutes.
Top each slice of toasted French bread with about 2 tablespoons of the mushroom mixture, place on platter and serve.
Recipe courtesy of Alex Guarnaschelli