Preheat oven to 180 degrees C (356 F) and line an oven tray.
Place the Portobello mushrooms, whole, on the lined tray and drizzle with a little olive oil and a pinch of sea salt. Pop in the oven to roast for around 12 minutes or until tender.
Meanwhile, combine the spice mixture in a small bowl.
Remove the roasted mushrooms from the oven to cool.
Add the drained black beans to a large bowl and mash them with a fork to break some of the beans up. This should only take about 10 to 15 seconds. Don't go overboard.
Next, place a ½ cup of the rolled oats in to a food processor and pulse for around 10 seconds or until the oats resemble a rough meal.
Roughly chop the cooked mushrooms and add them to the processor. Add the red onion and pulse again two or three times.
Add the remaining whole oats, the nutritional yeast, the spice mix, BBQ sauce, soy sauce, psyllium husk, chickpea flour, ¼ cup breadcrumbs (add up to ⅓ cup to help bind the burger), cooked lentils and mashed black beans.
Pulse together until the mixture comes together - you may need to stop occasionally to scrape down the sides and stir the mixture a little. When ready, the mixture will be wet but firm enough to bring together in to a pattie.
Remove the blade from the processor and line a large tray with greaseproof paper. Take a portion of the burger mixture, enough to fit in to the white of your palm, and shape it in to a patty. Place the finished patty on the lined tray and continue with the remaining mixture. I rinse my hands every so often - the mixture is a little wet and rinsing your hands just makes the process easier.
The patties are fine to go on the pan now or you can cover them and pop them in the fridge to further firm up.
To cook the burgers, heat a glug (around 1 tablespoon) of olive oil on a skillet or frypan over medium heat. When hot add 3 or 4 of the burgers to the pan depending on your pan size.
Cook the burgers on one side for 3-4 minutes or until the bottom is seared well before flipping it over to cook on the other side. Press the burgers gently with a spatula to flatten a little. Turn down the heat to low-medium and cook the other side for another 3-4 minutes. Repeat with the remaining burgers.
Serve on a burger bun with your favourite condiments. I love mustard, fresh lettuce, BBQ sauce and onion.
The burgers will keep for 3 to 4 days in the fridge in a sealed container or freeze for up to 2 months. If you are stacking the burgers to store, place a sheet of grease-proof paper between each layer so the burgers don't stick together.