Add onion, shallot, garlic, ginger, garam masala, turmeric, salt, red pepper flakes, and lemon zest to a blender; blend until smooth to create a fragrant curry paste.
Place cubed chicken and yogurt in the pressure cooker pot. Add curry paste and mix well to coat all pieces evenly.
Secure the lid and sealing ring. Set cooker to high pressure for 10 minutes. Let pressure release naturally for 10 minutes, then quick release.
Open lid and stir in coconut milk and tomato paste until smooth. Seal again and cook on low pressure for 5 more minutes.
Quick release, open lid, stir, and top with chopped cilantro. Serve hot over prepared rice or with naan.
Notes
You can double the curry paste and store for later. Freeze for up to 4 months or refrigerate for a week. Substitute Greek yogurt with coconut cream for dairy-free version.