Wash and brush the mussels under cold running water. Remove the beard tufts (if present) and sort out all open mussels, as they are inedible.
Peel onions and garlic. Dice the onions, cut the garlic into slices. Clean, wash and finely dice the carrots. Wash thyme and dab dry. Heat 15 g butter in a large pot, fry onions and garlic in it.
Add carrots, steam briefly and season with salt and pepper. Add 2-3 stems of thyme. Deglaze with white wine and lemon juice, bring to the boil and add the mussels to the stock. Simmer covered for about 12 minutes and stir in between.
Wash parsley and dill, dab dry and chop finely. Pour the mussels onto a sieve, drain and collect the stock. Keep the mussels warm. Heat the remaining fat in a pot and sauté the flour in it.
Add the mussel stock while stirring, bring to the boil and simmer for 4-5 minutes. Stir saffron and herbs into the sauce and season with salt and pepper. Arrange mussels on preheated, deep plates and pour the sauce over them.
Serve garnished with lemon and thyme. Serve with fresh white bread.