In a pan, melt the butter over medium heat.
Add the shallots, rosemary, and mushrooms.
Cook for 3 to 4 minutes, stirring regularly, until they have a nice golden color.
Add the broth and reduce over medium-low heat for 5 to 7 minutes, until the mixture is almost dry.

Put the warm mixture in a food processor and blend until you get a nice marmalade.
Add the cold butter cubes one by one, blending constantly until they are fully incorporated.
Adjust the seasoning with salt and pepper.

Spread each crostini with the marmalade.
Then top with a slice of smoked salmon and a slice of mushroom.
Drizzle with a squeeze of lemon juice, sprinkle with pepper, and serve.