Combine garlic, ginger, salt, cumin, turmeric, curry powder, paprika, and garam masala in a zip-top bag and shake to combine. Reserve 1 teaspoon of spice mixture. Add crema to the bag and mix to combine. Add chicken thighs to the bag and marinate in the refrigerator, 8 hours to overnight.
Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add 1 tablespoon butter and onions. Cook until browned and softened, about 6 minutes. Transfer to a small plate.
Add remaining butter to the pot. Pour in chicken thighs and marinade. Saute for about 2 minutes and hit Cancel. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 6 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Add rice to the pot around chicken thighs. Add caramelized onions. Pour coconut milk over rice, making sure rice is covered. Close and lock the lid. Select high pressure and set the timer for 8 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions for 30 minutes. Release remaining pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Add reserved spice mixture to the pot and crumble saffron on top of chicken and rice before serving.