Preheat the oven to 350°. In a small bowl, combine the tomatillos, red onion, cilantro and 1/2 teaspoon of the hot sauce.
In a medium bowl, combine the roast beef with the Fontina and the remaining 1/2 teaspoon of hot sauce. Arrange half of the tortilla chips on a large rimmed baking sheet. Spoon the roast beef filling onto the tortilla chips and top with the remaining 24 chips.
Bake the tostaditos for 5 to 7 minutes, or until the filling is hot and bubbling. Transfer to a platter. Season the tomatillo relish with salt, spoon a little on top of each tostadito and serve immediately.
Try a Sparkling Rosé from California or a fruity Gewürztraminer from Alsace.