While the grill is preheating, hand mix the ground beef with the meat seasoning. Be careful not to overwork the mixture. Form into four equal sized patties and set them aside.
Rub the poblano peppers with olive oil and place them on the grill directly over high heat. Turn them often in order to char the entire surface of the pepper. Once charred, remove them from the grill and seal in a sealable plastic bag, allowing them to steam for ten minutes. Then, peel off the outer skin, slice and remove seeds.
While poblano peppers are steaming, place a saucepan over medium heat (on the grill or stovetop) and melt the butter. Then, whisk in flour to make a roux, cooking over medium heat for three minutes. Add milk and cream, whisking for 2-3 more minutes until smooth and warmed through. Whisk in the remaining cheese sauce ingredients, and continue cooking until the mixture has melted smooth. About five minutes. Set aside and keep warm.
Season the Mexican burger patties with kosher salt, just before placing them on the grill directly over medium heat. Cook 3-4 minutes per side until desired internal temperature is reached as indicated by a reliable quick read thermometer.
Place a tostada shell on a bottom bun and top with a burger, roasted poblano pepper, queso fresco sauce, pico de gallo and fresh cilantro.