In a medium-sized pan, heat 1 tbsp oil. Place the onion into the pan and cook for 5 minutes, until translucent and soft
Add the mushrooms and garlic. Cook for 5 minutes more, until golden.
Stir in the rosemary and thyme. Cook for an additional minute.
Remove the pan from the heat and set to one side.
Place the drained black beans into a large bowl and mash until you’ve got a rough paste with small chunks of beans.
Stir in the shredded beet, mustard, mushroom mixture, salt and pepper. Mix thoroughly so all the ingredients are evenly dispersed.
Mix in the oatmeal flour.
Divide the mixture into 4 even pieces and form each piece into a patty shape.
Add 1 tbsp of oil into a large skillet over a low heat. Place the patties into the skillet and cook for 10-12 minutes on each side, until they’ve firmed up and keep their shape easily.
To assemble the burgers, top the bottom half of each burger bun with lettuce, patty, avocado pesto, tomato, onion, mayonnaise and the top burger bun half.