In a pan on medium heat, add 1 tablespoon olive oil with a third of the minced garlic. Add the chopped spinach with a light sprinkle of salt and pepper, and cook till it has wilted and darkened. Add to a bowl and set aside. Repeat with 1 tablespoon olive oil, another third of the garlic and the mushrooms with salt and pepper, and saute till the water has evaporated and the mushrooms have lightly browned. Add to the bowl with the spinach.
Now in the same pan, add the remaining 1 tablespoon olive oil with the last of the minced garlic, and add the diced onions. Fry till lightly golden, then add the remaining veggies along with the herb seasoning and another light sprinkling of salt and pepper. Cook till tender but not mushy. Taste and adjust seaoning. Stir in the cooked spinach and mushrooms and turn off the heat. As an option, I sometimes stir in 1/4th cup cream cheese after all the veggies are cooked, for a creamier filling. And you can also add the optional 1/2 cup between each layer like in step 5.
Now you're ready for assembly! Preheat the oven to 180 C.
Brush a light layer of olive oil all over the 9x5 loaf tin. Spread a very thin layer of the pasta sauce on the base and then place a lasagna sheet over it.
Spread about 1/3 cup of sauce over the sheet, followed by about 1/2 cup of the veggie filling. Sprinkle on about 1/3 cup of cheese and 2 to 3 teaspoons of cream cheese in small dollops over the veggies.
Over this, place a second lasagna sheet and repeat the sauce, filling and cheese combination until you have three layers with a total of four sheets. You may need to use a couple of extra sheets broken into pieces to fill up any gaps in the pan, depending on the size of the sheets themselves.
On the topmost lasagna sheet, spread the remaining sauce and sprinkle on the remaining cheese. Bake for about 45 minutes, until the cheese has turned lightly golden and melted, the liquid around the edges of the lasagna is bubbling away and a knife poked into the noodles goes straight through, indicating that they're cooked. If the cheese is browning too fast but the pasta sheets haven't softened, you can loosely cover the dish with foil and continue baking.
Let the lasagna rest for about 20 minutes to let the liquid get absorbed back in and for it to be easier to slice. Then use a sharp knife to cut into two large servings, or four smaller servings. Use a metal spatula to help take the slices out of the tin. Optionally, grate some parmesan and sprinkle some chopped basil over each slice. Dig in immediately! Leftovers can be kept in the fridge for 2 to 3 days and in the freezer for longer, reheating in the oven or microwave as needed. Happy cooking!