Preheat your oven to 200°C (400°F). Place the poblano peppers directly on a gas burner over medium-high heat, or on a baking sheet under a broiler. Roast, turning occasionally, until the skins are blackened and blistered on all sides, about 8-10 minutes. This charring is crucial for flavor.

Once roasted, immediately transfer the peppers to a heatproof bowl and cover tightly with plastic wrap or a clean kitchen towel. Let them steam for about 10-15 minutes. This steaming process loosens the skins, making them easier to peel.

While the peppers are steaming, prepare the egg custard. In a large bowl, whisk together the large eggs (about 300g / 10.5 oz total), milk (240ml / 1 cup), and evaporated milk (120ml / 1/2 cup) until well combined. This forms the base of your casserole.

Add the all-purpose flour (30g / 1/4 cup), salt (5g / 1 tsp), black pepper (1g / 1/4 tsp), and ground cumin (2g / 1/2 tsp) to the egg mixture. Whisk vigorously until the flour is fully incorporated and there are no lumps. The flour helps to thicken the custard slightly.

Once the peppers have steamed, carefully peel off the blackened skins. Make a slit down one side of each pepper and carefully remove the seeds and membranes. Be gentle to keep the peppers as intact as possible. You can rinse them briefly under cool water if needed, but avoid over-washing, which can remove flavor.

Lightly grease a 23x33 cm (9x13 inch) baking dish. Arrange the peeled and seeded poblano peppers in a single layer in the prepared baking dish. If using optional cooked shredded chicken or ground beef, or diced tomatoes, you can sprinkle these evenly over the peppers at this stage.

Evenly distribute about two-thirds of the shredded Monterey Jack cheese (about 135g / 1.5 cups) and all of the shredded cheddar cheese (100g / 1 cup) over the peppers and any optional fillings. This creates a cheesy layer within the casserole.

Carefully pour the prepared egg custard mixture evenly over the peppers and cheese in the baking dish. Ensure the custard covers all the ingredients.

Sprinkle the remaining one-third of the Monterey Jack cheese (about 65g / 0.5 cup) over the top of the casserole. If using optional chopped fresh cilantro, sprinkle it over the cheese now.

Reduce the oven temperature to 180°C (350°F). Bake the casserole, uncovered, for 30-40 minutes, or until the custard is set and a knife inserted near the center comes out clean. The top should be golden brown and slightly puffed.

Let the casserole rest for at least 10 minutes before slicing and serving. This allows the custard to fully set, making it easier to cut neat portions.