Preheat the oven to 450* F and line a baking sheet with parchment paper.
Toss the cauliflower, olive oil and 3/4 teaspoon of the salt together in a large bowl until the cauliflower is fully coated.
Spread the cauliflower onto the parchment lined baking sheet in one even layer and bake it for 20 minutes.
Add the onions to the large bowl and stir the vegetables around to coat them with any excess olive oil. If there is not enough remaining olive oil in the bowl simply add an additional 1/2 teaspoon.
After 20 minutes, add the onions to the baking sheet with the cauliflower and cook for an additional 10 minutes. Set aside.
Heat a stock pot or dutch oven over medium heat. Once the pot is hot add the butter. When the butter melts, add the garlic and cook for 1 minute until it is fragrant and lightly browned. Add the cream, remaining 1/4 teaspoon of salt and black pepper.
Raise the temperature on the stove to medium-high and heat for 2 minutes until the cream starts to bubble.
Add 2 cups of cheese into the cream and stir it until it is melted. Add the green chiles and stir to combine.
Add the roasted cauliflower and onion to the pot of cheese sauce and lightly toss to coat.
Add the cauliflower to a medium sized baking dish and sprinkle 1/4 cup of the shredded pepper jack over top. Don’t stress about the exact size of your baking dish, as long as you can get a solid layer across the bottom it will work. Bake for 10 to 15 minutes until it is bubbly. Serve immediately.