In the Instant pot --> PRESS SAUTE MODE --> PRESS ADJUST button and set on LESS heat.Add the butter and allow it to melt. Add the chopped garlic saute until the raw smell goes. Add the chopped onion and saute until it becomes slightly brown. Cover and cook for two minutes. Stir in between to avoid burning.
Add all purpose flour and stir until the flour gives a fried aroma. Add milk 1/2 cup first then another 1/2 stir well until there are no lumps. ( see notes)
Add red chili flakes, salt and pepper to taste. Stir well to a smooth consistency.
Add half the grated sharp cheddar cheese. Stir well.
Add cauliflower florets mix well.
PRESS CANCEL on Instant pot. PRESS MANUAL MODE ---> 4 MINUTES
After Instant pot finishes the 4 minutes --> IMMEDIATELY press CANCEL (SWITCH OFF)OPEN the lid after 15- max 20 minutes . (Natural pressure release)***DO NOT PUT LOW SETTINGS (after 4 minutes cook time) IT WILL BURN AS THERE IS NO WATER INSIDE.*** Gently stir ones and scoop the contents into a serving dish.
Sprinkle the remaining cheese on top and generously top it off with crispy fried onion
Serve the cauliflower casserole warm with more fried onion as desired