Combine yogurt and lemon zest and set aside.
In medium skillet, sweat leeks in 1 Tbsp olive oil over medium-low heat for approximately 2 minutes. Add thyme, salt, and pepper and continue to cook for a further 2 to 3 minutes, until leeks are soft. Set aside and keep warm.
Warm tortillas in toaster or oven. At same time, fry eggs in remaining 2 Tbsp olive oil.
Combine yogurt and leeks and spread over tortillas. Top each with egg and garnish with cheese and lemon zest. Serve immediately.