Slice the meat into thin strips and season with salt and pepper. Heat 1 tbsp oil in nonstick pan over medium-high heat. Add beef and stir-fry for 5 minutes, until browned but still pink in the centre. Remove from heat and keep warm.

Meanwhile, combine pumpkin seeds, olive oil, cider vinegar, avocado and salt in a food processor or blender container; blend until a pesto-like consistency.
Top each tostada with some lettuce, beef, refried beans, onion and tomato. Finish each with a spoon of the Mexican-style Pesto.