First, you’re going to prep the portobello mushrooms. First, hold the mushroom by the stem in your hand and use a damp paper towel to gently brush the dirt off the portobello mushroom caps. Then, carefully remove the stem and (optional) use a grapefruit spoon to gently scrape out the gills. Preheat the oven to 400 degrees. In a small bowl, add balsamic vinegar, low-sodium tamari, garlic powder, onion powder, and olive oil. Whisk it together.
Place the cleaned mushroom caps in a large baking dish that you lined with parchment paper. Drizzle the sauce over the mushrooms and then use your hands to rub the sauce all over the mushrooms. Let the mushrooms marinate for 20 minutes.
After 20 minutes, use your hands to rub any of the sauce that dripped off back onto the mushrooms and then place the dish into the oven to bake for 20-25 minutes (depending on how big the mushrooms are), flipping halfway through.
While the mushrooms are roasting, cook the green lentils in salted boiling water for 18-20 minutes or until tender. Drain the excess liquid and cover the pot to keep the lentils warm until everything else is ready.
While the lentils and mushrooms are cooking, make the green pea pesto sauce. Add fresh basil, green peas, pumpkin seeds, lemon juice, olive oil, water, garlic cloves, salt, and pepper to a food processor or blender. Blend until smooth. Pour the pesto sauce into a pan and warm it up over medium heat.
When the mushrooms are done roasting, remove the tray from the oven and let them cool for a few minutes before you try to slice them. Once cool enough to handle, slice the mushrooms into 1″ thick slices.
To serve, pour a little bit of the warm pesto sauce onto a plate and then top with a heaping scoop of the cooked lentils and one whole sliced portobello mushroom. Garnish with chopped parsley or chives.