Spiced red lentil and tomato soup

Ingredients

• 1 large onion
• 2 cloves garlic
• 1 tsp garam masala
• 1 tsp cumin powder
• 1 tsp ground coriander
• 1 tsp tumeric
• 2 tins chopped tomatoes
• 400ml water
• 200g red lentils
• Salt and pepper to taste
• Small tin coconut milk (optional)