Heat 1 tablespoon olive oil in a dutch oven or large, high-sided sauté pan over medium heat. Add the mushrooms, onion, half of the garlic, 1/2 teaspoon salt, and 1/4 teaspoon fresh ground black pepper. Sauté for 10-12 minutes or until the mushrooms and onion release most of their water and the pan returns to mostly dry. Once done, transfer the mushroom mixture to a small bowl and return the pan to the stovetop.
Tent a piece of aluminum foil over the lasagna and let it bake in a pre-heated 375° oven for 35-40 minutes. Then remove the foil and return the lasagna to the oven for an additional 12-15 minutes. Once done, take the lasagna from the oven and let it sit at room temperature for at least 10-15 minutes before cutting into it or serving. This will allow the pasta and vegetables to re-absorb some of the juices and help keep your lasagna stacked high. You may freeze the lasagna (uncooked) for up to 1 month if adequately covered and protected from freezer burn. See TIP C below.
TIP A: I've said this before (in my pizza post), but I can't stress it enough! When you're using a store-bought tomato sauce as a base for lasagna, pizza or something else where you're going to be adding a lot of flavor, keep it simple! A lot of jarred "marinara" and "pizza" sauces have strong herb-y flavors that can compete with the flavors that you're actually trying to achieve in your dish. My favorite simple tomato sauce is Cento® Tomato Sauce, but any variety made with just high-quality vine-ripened tomatoes and a pinch of salt is ideal.
TIP B: If you haven't done so already, then I highly recommend making a jar of Homemade Za'atar to be your good-on-anything seasoning of 2018. That said, if you just want to get this lasagna in the oven or freezer and be done, then you can prep this easy substitute: 1 teaspoon fresh lemon zest, 1 teaspoon dried thyme leaves, 1 teaspoon dried oregano leaves, and 1/2 teaspoon kosher salt. Use all of the mixture in the place of the recommended 1.5 tablespoons za'atar.
TIP C: Lasagna is a great dish (maybe the best ever) to make-ahead, freeze, and bake later. The trick is to ensure that you adequately protect the lasagna from freezer burn (i.e. exposure to air) while in the freezer. To do so, first wrap the entire dish all the way around in plastic wrap and ensure that the wrap touches the top of the lasagna to the corners and edges. Then cover the plastic wrap with a layer of aluminum foil—again, all the way around—to form a second protective barrier. Bake the lasagna within 2 weeks of freezing. Allow approximately 48 hours for the lasagna to fully defrost in the fridge before baking. Unwrap the plastic wrap layer and bake according to the instructions above.