Place the wild rice, brown rice, and vegetable broth into a small saucepan. Set it over high heat. Bring the liquid to a boil, lower heat to simmer, and cover. Allow the mixture to continue simmering for 50 minutes, until the rice is tender. Drain any excess broth when done, then let the rice sit in the pot, covered, until ready to use.
While the rice mixture cooks, preheat the oven to 400°F and line a baking sheet with parchment paper.
Optionally, use a spoon to scrape the gills out of your portobello caps (Note 1). Brush the mushrooms lightly with olive oil and bake them for 15 minutes, until they just begin to soften. Blot any moisture that has collected inside of the mushroom caps when they come out of the oven.
Prepare the filling while your mushrooms cook. Heat 1 tablespoon of olive oil in a medium skillet over medium heat. Add the onion to the skillet. Cook the onion for 5 to 10 minutes, stirring occasionally, until softened. Stir in the garlic and cook it with the onion for about a minute, until very fragrant.
Add the cooked rice mixture, rosemary, walnuts, nutritional yeast, and lemon juice to the skillet. Cook and stir the mixture until heated throughout.
Remove the skillet from heat. Mix in the parsley, and season the mixture with salt and pepper to taste.
Divide the rice mixture among the portobello caps.
Serve.