Side to serve with spinach mushroom lasagna?

I like the Ottolenghi salad with beets, rocket, and sunflower seeds in a maple vinaigrette. I make the dressing less sweet compared with how it is written (I use equal parts maple syrup, olive oil, and sherry vinegar). I like this recipe with cheesy mains because it doesn't have any cheese in it. https://jonoandjules.com/2011/10/25/ottolenghi-roasted-beetroot-salad/

You can never go wrong with a simple Caprese salad or antipasto. That said, your lasagna sounds absolutely divine! Would you be willing to share your recipe? I would love to try! Thanks & good luck with your party!

Since you already have greens in your lasagna, how about a French-style shredded carrot salad? Its tanginess and crunchiness, as well as its color, would contrast with the lasagna really well. Here are some (very simple) instructions for it on David Lebovitz's blog: https://www.davidlebovitz.com/carottes-rapee/

In Italy they have a taste for bitter greens from rochetta (arugula) to radicchio to broccoli rabe (rapini). Any of these would add some balance to the lasagna you described, either alone or in a salad mix. In cooking greens they tend to "hammer" them with long cooking which coaxes out all the flavors.

Kale salad with (shaved) Parmesan cheese and a bright, punchy lemon vinaigrette. You could add in breadcrumb as a crouton but you noted you'll be serving garlic bread on the side already!

You might serve nibbley bits - a sort of antipasti. I was thinking of really good olives etc. The main will be substantial.

One or more of:
Sliced oranges and red onions, olive oil & red wine vinaigrette.
Black and/or green olives liberally doused with lemon zest, garlic & fresh thyme.
Pickled beets.
Red cabbage slaw.
Sliced cucumbers in clear or sour cream dressing.
Roasted herbed skinny carrots, served hot or cold.

I would go for a nice crisp salad with a lemony vinaigrette.