Heat a thin layer of oil in a small skillet and fry the tortillas 1 at a time over medium-high heat until golden and crisp. Season with salt.
In a medium nonstick skillet, heat a drizzle of oil over medium-high heat. Add the beef and brown for about 5 minutes. Add half each of the onion and jalapeno, the garlic, chili powder, cumin and coriander; season with salt and pepper. Cook until the onion is softened, about 5 minutes. Stir in the tomato paste for 1 minute. Add the beef stock and lower the heat to a simmer.
In a small bowl, combine the tomatoes with the remaining onion and jalapeno and the cilantro. Season the salsa with salt.
Heat the butter in a small saucepan over medium heat. Whisk in the flour for 1 minute. Whisk in the milk and cook to thicken, about 2 minutes. Stir in the cheese to melt.
Place a tortilla on each dinner plate. Cover with the meat mixture and cheese sauce, then top with the tomato salsa, pickled jalapenos and sour cream.