Preheat your oven to 400°F (205°C). Prepare a baking sheet by covering it with parchment paper, and brushing it with olive oil.
Toss the chopped mushrooms and walnuts in a bowl with olive oil and roast them on the prepared pan for 18 minutes until the mushrooms are tender, and slightly wrinkled looking.
While the mushrooms are roasting, put the oats, dried porcini (if you are using them), onion, garlic, salt, pepper, coriander, red pepper flakes (if you're using them), oregano, thyme, and toasted fennel seeds into a food processor. Give it a few pulses until it's all coarsely ground.
Now, take the roasted mushrooms and add them to the food processor, along with the black beans and tomato paste. Pulse everything together until it's well mixed. You want to keep some chunky bits of mushrooms for texture.
With your hands, make small balls from the mixture. Keep a bowl of water on hand to keep wetting your hands as you roll the balls so that the mixture doesn’t stick to your hands.
Over medium-high heat, warm a skillet with 4 teaspoons of olive oil. After 2 minutes, when the oil is hot, put the meatballs in the skillet. Cook them for 5-6 minutes, turning them or shaking the pan every 30 seconds or so, so that the meatballs brown lightly on all sides.
After they've browned, move the skillet into the oven. Bake them for another 18-20 minutes. They should be firm and a bit crisp on the outside when they're done.
Take them out and allow them to cool to room temperature. Once you can handle them comfortably, give them each a gentle squeeze in your hands to compress them before you ladle on marinara, or the sauce of your choice and garnish with fresh herbs.