Crush nori sheets with a small food processor or use the chopping bowl of an immersion blender.
Soak cauliflower florets in hot water for 10 minutes. Drain them and season them with salt.
Make the dry batter by mixing 1/2 cup rice flour, 2 Tbsp cornstarch, and 1/4 tsp salt.
Make the wet batter by mixing chickpea flour, 2/3 cup rice flour, 1/2 tsp salt, 1/8 tsp pepper, 1/2 tsp garlic powder, 1/2 tsp smoked paprika, and 2 tsp crushed nori sheet. Start adding milk and whisk until you get the right consistency.
Take a deep frying pan and heat vegetable oil over high heat. To test the temperature, drip a bit of wet batter. If it is sizzling, the oil is ready for deep frying.
Start dipping cauliflower florets first into the wet, then into the dry batter.
Carefully place the battered florets in batches into the hot oil. Don't overcrowd the pan. Fry each batch for 4-6 minutes. It should have a light brown color.
Add vegan mayo, minced garlic, sriracha, and 1 Tbsp lemon juice in a small bowl. Stir well.
Assemble the tacos by adding the fried cauliflower florets, freshly chopped tomatoes, and onion. Drizzle some spicy mayo sauce on top.