Heat a medium saucepan over medium heat, add the butter and allow to melt. Add the flour, whisking to combine. Let the roux cook for a minute until a pale golden brown. Add the milk, whisking to incorporate.
Bring the mixture to a simmer, then lower the heat to medium-low, whisking until the sauce thickens, about 2 minutes. Slowly add 1 cup of the cheddar, the Monterey Jack cheese and salt, whisking to combine, until the cheese is completely melted.
Preheat oven to broil, high. Spread the chips on a baking sheet and ladle cheese sauce over the top. Spread the remaining Cheddar cheese over the top and broil until the Cheddar has melted, about 1 minute. Remove from the oven and spread the chips topped with cheese on a large platter.
Top with the grilled chicken, beans, green onions, queso fresco, pico de gallo, avocado, jalapeños and sour cream.
Serve immediately.
Notes
Layer chips, seasoned chicken, beans, and cheese evenly so every bite gets a mix of crunchy chips and melted cheese instead of a clump in one spot.