In a Dutch oven or large skillet, over medium heat, melt butter. Add minced onion and salt, cook about 5 minutes until onions are softened, but not browned.
Stir in the black pepper, coriander, cumin, ginger, cinnamon, and cayenne. Cook and stir for about 2 minutes to bloom the spices.
Stir in tomato paste and minced garlic. Cook for 1 minute, stirring frequently.
Add chicken broth, water, and lentils; heat to rapid simmer, stirring frequently. Simmer about 15 minutes, until lentils are soft and starting to break down; continue stirring frequently. Stir in lemon juice.
Whisk soup with a wire whisk to break down lentils and create a thicker soup. There will still be some lentils and small pieces that will give the soup texture.
Spiced Butter Drizzle
Melt 2 tablespoons unsalted butter in a small saucepan. Take off heat. Stir in dried mint and paprika.
To serve:
Top soup with a drizzle of the spiced butter and some chopped cilantro.