For the cheese sauce: Add the butter to a medium, heavy-bottomed saucepan over low heat. As the butter melts, add the flour and slowly mix to make a roux. Cook for a few minutes, then add the cream and salt and let simmer on low heat until thickened, a few minutes more. Slowly fold in the mixed cheeses and keep warm.
For the rib-eye mixture: Heat a large, heavy-bottomed saucepan, then add the oil along with the onions and jalapenos. Saute until the onions are translucent, 3 to 4 minutes, then add the tomato paste and continue to cook, stirring occasionally, 1 to 2 minutes. Add the beef, breaking it up with a spoon, and a big pinch of salt. Cook until the beef is cooked through, another 8 to 10 minutes. Season to taste with salt and pepper. Set aside.
For the smashed avocado: Scoop and smash the avocado flesh into a medium bowl, then add the cilantro and lemon juice. Season to taste with salt.
For the nachos: Spread some chips on a large platter. Top with some cheese sauce, then some meat. Repeat the layering. Top with the smashed avocado and garnish with shredded Cheddar. Serve.