Preheat your oven to 350°F or 175°C (350°F). Lightly coat a medium saucepan with nonstick cooking spray and heat over medium-high.
Add the chopped onion and minced garlic, cooking and stirring until the onion is soft and fragrant.
In the same saucepan with the aromatics, prepare the rice pilaf and lentil mix according to the package directions. This infuses the grains with flavor right from the start.
While the pilaf cooks, drain the marinated artichoke hearts, making sure to reserve the marinade. Coarsely chop the artichokes and set them aside. Gently snap off the mushroom stems so they are even with the caps; you can discard the stems. If you prefer a smoother texture, carefully scrape out the dark gills from the undersides of the caps with a spoon.
Arrange the mushroom caps, stemmed side up, in an ungreased shallow baking pan.
Brush them generously with some of the reserved artichoke marinade.
Discard any leftover marinade.
Bake for 15 to 20 minutes, or until the mushrooms are tender and have released some of their liquid.
Once the pilaf is cooked, stir in the chopped artichokes and the optional Parmesan cheese until well combined. Carefully transfer the roasted mushroom caps to a serving platter. Generously spoon the hot pilaf mixture into each mushroom cap and serve immediately.