Heat 1 tablespoon oil in a large pot over medium-high heat.
Add chicken.
Add fenugreek and garam masala. Season with salt and pepper.
Sauté, stirring, for 4-5 minutes or until chicken gets a light brown color. Remove from pan.
Add remaining oil to the pan.
Add onions, ginger and garlic. Cook, stirring occasionally, for about 2-3 minutes or until the onions are soft.
Add cumin, garam masala and red chili powder. Sauté, stirring, for about one minute or until fragrant. Stir in butter and lemon juice.
Add tomato paste. Cook, stirring, for one minute.
Using an immersion blender, blend the sauce to a smooth puree.
Add cream and yogurt. Season with salt and pepper.
Bring the soup to a boil. Simmer for about 3-4 minutes.
Stir in cashew nut paste if using. Cook for another 4-5 minutes.
Divide soup among serving bowls.
Garnish with coriander (cilantro) leaves, cream and chopped nuts.