Prep and cook kumara 'buns'. Pre-heat oven to 220°C. Toss kumara rounds on a lined oven tray with a drizzle of olive oil. Season with salt and roast on lower oven rack for 25-30 minutes, until golden and tender.
Prep and cook thyme roasted tomatoes. Arrange tomato sliced on a lined oven tray. In a small bowl, combine thyme leaves and garlic with 2 Tbsp olive oil. Drizzle thyme and garlic oil over tomatoes, season with salt and pepper and roast on rack above kumara for 20-25 minutes, until golden.
Prep and cook black bean patties. Open the can of black beans and reserve 2 Tbsp of the brine in the can. Drain black beans and rinse with cold water. Place drained and rinsed black beans into a bowl and mash with a potato masher or fork. Add the 2 Tbsp of reserved black bean brine and the remaining patty ingredients to bowl along with ½ tsp salt. Mix until well incorporated (use your hands to mix everything together). Shape ¼ cups worth of the mixture into patties about 2 cm thick (they should be roughly the same size as the kumara rounds).
Heat a drizzle of oil in a large fry-pan on medium heat. Cook patties for 3-4 minutes each side, until browned and hot through. Add more oil to pan between batches when needed.
Serve kumara rounds filled with the black bean patties, roasted tomatoes, rocket, red onion, avocado and a drizzle of vegan lemon herb mayo (or another vegan mayo alternative).