Slow Cooker Chicken Tostadas

Name: Slow Cooker Chicken Tostadas
Ingredients: 2 white onions, halved; 1.50 pounds boneless, skinless chicken thighs; 1 16-oz. jar tomatillo salsa; 2 poblano chiles, seeded and cut into 1-in. pieces (about 2 cups); 1 teaspoon kosher salt, divided; 3 large plum tomatoes, diced (2 cups); 3 tablespoons finely chopped fresh cilantro leaves; 2 tablespoons fresh lime juice (from 1 lime), plus wedges for serving; 8 corn tostadas; 2.50 ounces queso fresco (fresh Mexican cheese), crumbled (about ½ cup)