1. Place a medium saucepan over medium heat. Add butter and stir until melted. Add flour, then cook for 2 minutes, stirring constantly. While whisking briskly, slowly pour in the milk a little at a time until it is fully incorporated.
2. Bring the milk mixture to a boil, whisking frequently. Remove from heat, add the blue cheese, and whisk until melted. Taste and season with salt and pepper, as desired. Cover to keep warm and to prevent a skin from forming on the sauce.
3. Place a large, heavy skillet over medium-high heat. Add the oil followed by the ground beef. Season generously with salt and pepper. Cook the meat, breaking it up as you go, until it is mostly cooked, about 5 minutes.
4. Add the diced white or yellow onion to the ground beef and continue cooking until the onions become tender and the beef is fully cooked, about 3 minutes more. Remove from the heat, cover and set aside.
5. Evenly distribute the tortilla chips among 4 large plates. Pour an even amount of the blue cheese sauce over the chips followed by the beef, crumbled blue cheese, chopped tomato, chopped parsley, jalapenos and onion. Serve immediately.