Preheat your oven to 400°F and place the corn tortillas on a baking sheet, cooking for 8-10 minutes until they are crisp and golden.
Season the chicken breast with cumin, chili powder, garlic powder, sea salt, and black pepper.
Poach the chicken in simmering water or pan-sear until the internal temperature reaches 165°F, then shred finely using two forks.
Toss the shredded chicken with half of the lime juice to keep it moist and flavorful.
In a small bowl, mash the avocado with the Greek yogurt and the remaining lime juice until a smooth, velvety crema forms.
Spread a generous layer of avocado crema onto each crispy tortilla, then top with the shredded chicken.
Finish the tostadas by garnishing with shredded romaine lettuce, thinly sliced radishes, and fresh cilantro.