Roughly smash beans with potato masher or back of wooden spoon; set aside.
Place 9 in. (23 cm) cast-iron skillet on grill; bring to medium-high heat. Add oil. Stir in cauliflower, mushrooms and walnuts. Cook 5 to 8 min., until browned. Stir in smashed beans, 4 green onions, hot sauce, lime juice and soy sauce. Cook, stirring, 3 to 5 min. to combine ingredients. Remove from heat. Garnish with additional green onion.
Toast taco shells 1 to 2 min. per side on grill. Spoon in filling. Top with choice of lettuce, tomato, salsa, jalapenos, vegan sour cream or cheese.
Alternatively, cook taco filling on stovetop.