Heat a large skillet over medium-low heat. Add olive oil and butter, then add mushrooms and toss well to coat. Cover and cook for 8-10 minutes, until soft and juicy, stirring once or twice during cooking time.

While mushrooms are cooking, whisk together dijon mustard, egg yolk and lemon juice in a large bowl until combined. Slowly stream in olive oil while continuously whisking until the mixture emulsifies and comes together completely. Whisk in whole grain mustard and horseradish, then taste and season with salt and pepper as desired.

Heat the broiler on your oven to the highest setting, placing an over rack directly underneath. Lay four slices of whole grain bread on a baking sheet and pop under the broiler for 1-2 minutes, until golden. Remove and spread a layer of mustard aioli on top.

Remove the cover from the mushrooms and stir in the garlic, salt and pepper, cooking for one minute. Turn off heat, then distribute mushrooms evenly among the four slices of toast. Cover with grated cheese. Place back under the broiler for 1-2 minutes, until cheese is bubbly and slightly golden. Remove and serve immediately with extra mustard aioli on top.