Place the mussels in lukewarm water for about 10 minutes so that they open and the sand is rinsed out. Then rinse under running tap water so that the remaining sand is also removed. Place the mussels side by side on a baking sheet.
Finely dice the shallots and garlic. Chop the parsley.
Put the olive oil in a pan and sauté the shallots with the garlic. Season with salt, pepper and chilli flakes. Spread the mixture evenly over the mussels and place in the oven for approx. 4 - 5 minutes directly under the preheated grill (approx. 230 degrees).
Sprinkle with parsley, drizzle with lemon juice and serve immediately.