Melt the butter in a large saucepan and sauté the onion, garlic and zucchini gently for about 10 minutes, stirring once or twice. Do not let them brown.
Add the spinach and cook until wilted then pour in the broth. Bring to the boil, season to taste then stir in and simmer, partially covered, for about 10 to 15 minutes.
Strain the vegetables and reserve the liquid. Pass the vegetables through a food processor or blender, gradually adding back the liquid and adding 2 tablespoons of the quark. Return the soup to the pan and set aside.
Either cut the pumpernickel into small rounds using a cookie cutter or leave whole. Toast under a hot grill for 1 to 2 minutes until just crisp. Remove, cut into quarters if not already cut into rounds, and allow to cool.
Whisk the herbs and some seasoning into the rest of the quark. When ready to serve, spread the quark on the pumpernickel toasts. Bring the soup to the boil and check the seasoning, adding a little lemon juice if you think it needs it. Divide the soup between warmed bowls and top or serve with the pumpernickel toasts.