1. Melt 30g (2 tablespoons or 1.1 oz) of unsalted butter or margarine in a large pot over medium heat, roughly 180°C (356°F).

2. Add 1 medium chopped yellow onion to the pot and sauté until softened, about 5 minutes.

3. Add 5g (1 teaspoon) of black pepper, 5g (1 teaspoon) of salt, and 2 vegetable bouillon cubes to the pot. Stir well to combine.

4. Pour in 850ml (28.7 fl oz US) of water and bring the mixture to a boil. Reduce the heat to low, roughly 90°C (194°F), and simmer for 10 minutes.

5. Add 250g (8.8 oz US) of frozen green peas to the pot and cook until they're tender, about 5 minutes.

6. Stir in 200g (7 oz US) of frozen baby spinach and cook until wilted, about 2 minutes.

7. Remove the pot from the heat and stir in 30ml (2 tablespoons US) of freshly squeezed lemon juice. Let the soup cool slightly before serving.