Preheat the oven to 400°F (200°C). Line a large baking sheetwith parchment paper.
Using 1 tablespoon of the olive oil, brush the tops andbottoms of the portobellos until each is completely coated.Place them stemmed-side down on the prepared baking sheetand sprinkle each with a pinch of salt. Bake for 20 minutes,or until the caps look like they are shriveling up and are fork-tender (very large mushrooms may need 5 minutes more).
Meanwhile, in a medium skillet, heat the remaining 1 tablespoon olive oil over medium heat. Add the onion and garlic and stir to coat with the oil. Sauté for 5 to 6 minutes, until the onion issoftened.
Chop the walnuts into pieces just smaller than pea-sized.Add the walnuts, 2 teaspoons of the rosemary, the parsley,drained lentils, vinegar, and salt (I use about 1⁄2 teaspoon) and pepper to taste to the onion mixture in the skillet and stir well to combine. Taste and adjust the seasonings, if desired. Cook fora few minutes, until heated through. Taste and stir in more ofthe rosemary, if you’d like a stronger rosemary flavor.
Make the Balsamic-Apple Glaze: In a small pot, whisk together the ketchup, vinegar, applesauce, and maple syrup and heat over medium-low heat, stirring every now and then, until warmed through.
Place the roasted portobellos stemmed-side up, like a bowl, and divide the crumble evenly among them (I use about 1⁄2 cup crumble on the large portobellos and¼ on the smaller ones). Season with a sprinkle of salt and pepper and serve immediately, drizzled generously with the warm glaze.