Makes 4 servings
Heat grill to low. Combine cumin, coriander, ground mustard, and paprika in a small bowl. On a large plate, combine 3 teaspoons spice mixture and oil. Coat cauliflower steaks in oil mixture. Grill, turning occasionally, until nicely charred and crisp-tender, about 15 minutes. Combine remaining 1 teaspoon of spice mixture and Vegenaise. Spread mixture on each bun and top with grilled cauliflower, cilantro, and cucumber.