Creamy pea soup

Name: Creamy pea soup
Prep time: 10 min
Cook time: 40 min
Total time: 50 min
Cuisine: European, British
Ingredients: 150 g onions, cut into halves; 20 g butter; 1000 g fresh green peas or 1000 g frozen green peas; 600 g water; 1 tbsp chicken stock paste, homemade or 1 chicken stock cube (for 0.5 l); 200 g crème fraîche or 200 g sour cream; 2 pinches freshly ground black pepper, to taste; ½ tsp fine salt, to taste