Bring the fish, peeled and halved onion together with bay leaves, parsley, juice of one lemon, white wine in 1 1/2 liters of water to a boil and simmer gently for 30 minutes. Add peppercorns and let the clear soup simmer for another 5 minutes. Then pour through a sieve and let the clear soup bubble repeatedly. Add the paradeis pulp, paprika and saffron and continue to simmer until the mixture has reduced by a third.
For the rouille, add the egg yolks, half of the vinegar and the mustard and stir well. Add the oil drop by drop at the beginning and then a little bit faster until the mixture has a creamy consistency. Next, add the remaining vinegar. Color this mayo red with paprika powder and season with squeezed garlic cloves and cayenne pepper.
Spread each two toasted baguette slices with rouille and place in a soup plate, pour fish soup over them and sprinkle with Gruyère cheese.