Set Instant Pot to Sauté Mode, Normal Heat. Melt butter in Instant Pot.
Add onion. Cook until softened, approximately 4 to 5 minutes.
Reduce heat to Low Heat. Add serrano chiles, garlic, and ginger. Cook approximately 10 minutes, stirring occasionally, until mixture develops a nice, even, toffee-brown color. Should mixture start to dry out or stick, add water as needed, 1 to 2 teaspoons at a time. If cooking over Normal Heat instead of Low, monitor the mixture closely, or it will burn.
Once desired color is achieved, add garam masala and paprika. Stir well. Cook 1 to 3 minutes, or until fragrant. Should mixture start to dry out or stick, add water as needed, 1 to 2 teaspoons at a time.
Add tomato sauce. Stir well.
Transfer mixture to blender and blend until smooth. See Notes for tips on blending hot liquids.
Return mixture to Instant Pot and set heat to Normal Heat. Simmer 15 to 25 minutes or until thick and paste-like.
Add chicken (and its marinade), sliced bell pepper, fenugreek leaves, and salt. Secure lid. Cook on Manual High Pressure for 6 minutes. When time is up, carefully Quick Release.
Add heavy cream. Stir until smooth. Taste and adjust seasonings, adding more salt or fenugreek leaves as necessary.
Divide into portions. Optional: Garnish with chopped cilantro. Serve over basmati rice with naan.