Make the slaw. In a food processor, combine the avocado, yogurt, limes, cilantro, garlic, chile, and salt. Add ¼ cup water and purée until smooth. Add a little more water as needed, 1 tablespoon at a time, to loosen. Taste and adjust the seasoning as needed.
Pour half the dressing into a large bowl. Add the cabbage and toss to coat.
Make the fish. Place the egg in a shallow medium dish or large plate. Spread out the quinoa in a separate shallow medium dish and season with salt. Dip the fish in the egg, letting any excess drip back into the bowl, then roll it through the quinoa to coat. Repeat to coat the remaining fillets.
Heat the olive oil in a large skillet over medium-high heat. When the oil is shimmering, add the coated fish and cook until well browned on the bottom, 2 to 3 minutes. Flip the fish and cook until browned on the outside and cooked through, 2 to 3 minutes more. Transfer to a paper towel–lined plate to drain.
In a large bowl, whisk together the olive oil, lime juice, honey, and cilantro. Season with salt and pepper. Add the lettuce and toss to coat.
Divide the lettuce among four bowls and top each with a fish fillet, some mango, and some slaw. Serve, passing the remaining dressing at the table.