1. Rinse salmon and pat dry; cut the fillet crosswise into 1/2-inch-wide strips (cut any longer than 6 in. in half). In a bowl, whisk flour, beer, egg, and mustard until well blended.
2. Pour 1/2 inch oil into a 10- to 12-inch frying pan over high heat. When surface is rippling, one at a time, dip salmon strips in beer batter, turn to coat all sides (or spoon batter over fish), and lay slightly apart in a single layer in hot oil. Cook, turning pieces once with a wide spatula, until batter is golden brown on all sides and salmon is just opaque but still moist-looking in center of thickest part (cut to test), about 3 minutes total. As cooked, transfer to a paper towel-lined 12- by 17-inch baking pan and keep warm in a 200° oven. As there's room in frying pan, coat and cook remaining salmon strips. Add to fish in baking pan and sprinkle all with salt. Discard remaining batter and the oil in pan; wipe pan clean with paper towels.
3. Set frying pan over medium-high heat. One at a time, lay tortillas in pan and heat, turning once, just until warm, about 30 seconds total for each tortilla. As heated, transfer to a platter or plates and fill each with an eighth of the salmon and 1/4 cup shredded cabbage. Add chipotle mayonnaise and tomatillo salsa as desired.
4. Serve with lime wedges to squeeze over tacos and remaining mayonnaise and salsa to add to taste.
Nutritional analysis per taco.