To make the slaw, combine the cabbage, tomato, onion, cilantro, and lemon juice in a large bowl. Add the mayonnaise, using however much you like in your slaw. Season with salt and pepper to taste. Toss until evenly coated, then taste and adjust seasonings.
Slice the tempeh into strips.
To make the batter, combine the flour, potato starch, seafood seasoning, dulse flakes, and baking soda in a bowl and mix well. Add the vinegar and seltzer, and stir until just mixed. The batter will have the consistency of a fluffy pancake batter.
Heat about 1 inch of oil in a pan. Dip the tempeh pieces into the batter to coat. Fry the tempeh in the pan, turning once, until crispy and golden, about 3 to 5 minutes per side (see note).
While the tempeh is cooking, warm the tortillas in a skillet over medium heat to soften. Stuff the tortillas with the tempeh and slaw and serve hot with all the usual taco fixings.