1. Chop fish into 1/2 cubes; place in medium bowl. Add kosher salt; toss to coat. Add lime juice; toss to coat. Marinate until the edges of the cubes begin to turn opaque, tossing occasionally, about 30 minutes.
2. Dice avocados; add to bowl along with green olives, tomatillos, onion, cilantro, and jalapeño Add olive oil and season to taste with salt.
3. Serve over tostadas or with tortilla chips for dipping.
4. Per serving: 244.1 calories, 151.8 calories from fat, 16.9 g fat, 2.0 g saturated fat, 24.2 mg cholesterol, 7.8 g carbohydrates, 3.8 g dietary fiber, 1.3 g total sugars, 4.1 g net carbohydrates, 17.0 g protein, 472.1 mg sodium Nutritional analysis provided by Bon Appétit