Cut mushrooms in quarters, slice onion and keeping garlic cloves in shell, smash them slightly with a back of a water glass.
Mix mustard, 3 tablespoons olive oil and white wine vinegar. Add a pinch of salt and black pepper.
Arrange mushrooms, onion slices and garlic cloves on a baking tray. Sprinkle with the sauce and roast for 20 minutes.
Remove garlic cloves from shells, add the rest of olive oil and dice everything together with a Tupperware chopper or blender. The mixture should have a grainy consistency of caviar. Season with salt and pepper to taste.
Serve over rye bread, crackers or Russian pancakes. Optional: sprinkle with chopped chives. Enjoy!
Keep in an airtight jar in the fridge for up to 1 week.