If using fresh poblano peppers: Preheat your oven to 400°F (200°C) or prepare your broiler. Place the poblanos on a baking sheet. Roast for 15-20 minutes, turning occasionally, until their skins are blistered and charred. Alternatively, broil for 5-7 minutes, turning once, until charred. Transfer the hot chiles to a bowl, cover tightly with plastic wrap, and let them steam for 10-15 minutes. Once cooled, carefully peel off the skins, remove stems and seeds, and roughly chop the peppers into 1-inch pieces. If using canned diced green chiles (2 cans, 7 oz each), simply drain them well.

Lightly grease the inside of your 6-quart slow cooker with olive oil or cooking spray.

Spread half of the chopped chiles evenly on the bottom of the slow cooker. Sprinkle with half of the shredded Monterey Jack and cheddar cheeses. Repeat with the remaining chiles and cheeses, creating two distinct layers of each. If using, sprinkle the diced white onion over the chile and cheese layers.

In a large mixing bowl, whisk together the eggs, milk, flour, salt, pepper, ground cumin, garlic powder, and onion powder until the mixture is well combined and smooth.

Carefully pour the egg mixture evenly over the layers of chiles and cheese in the slow cooker. Gently shake the slow cooker a few times to ensure the egg mixture settles into all the crevices.

Cover the slow cooker and cook on LOW for 3 to 4 hours, or on HIGH for 2 to 2.5 hours. The casserole is done when the center is set, and a knife inserted near the center comes out clean. The edges should appear golden and slightly bubbly.

Once cooked, turn off the slow cooker and let the casserole rest, covered, for 15-20 minutes before serving. This resting period allows the casserole to set further, making it easier to slice and serve.

Garnish individual servings with chopped fresh cilantro, a dollop of sour cream, and a side of salsa, if desired. Serve warm.