Red Lentil Soup with Lemon

INGREDIENTS

• 2 cups / 14 oz / 400 g split red lentils, picked over and rinsed well
• 1 tablespoon turmeric
• 4 tablespoons unsalted butter
• fine grain sea salt
• 1 large onion / ~ 2 cups, diced
• 2 teaspoons ground cumin
• 1 1/2 teaspoons yellow mustard seeds
• 1 cup chopped cilantro
• Juice of three lemons, or to taste
• 1 large bunch of spinach leaves, chopped
• plenty of cooked (warm) brown rice, to serve
• plenty of plain Greek yogurt, to serve