Before starting, preheat the oven to 450°F. Wash and dry all produce. Pull stems off mushroom caps, then discard. Brush mushroom caps all over with 2 tsp oil (dbl for 4 ppl). Season with 1/4 tsp garlic salt (dbl for 4 ppl) and pepper. Arrange mushroom caps on a parchment-lined baking sheet, top-side up. Roast in the middle of the oven until mushrooms are juicy and fork-tender, 10-12 min.

Meanwhile, slice 1 tomato (2 for 4 ppl) into 1/2-inch rounds. Cut 1 tomato (2 for 4 ppl) into 1/2-inch pieces. Halve buns.

Stir together mayo and half the mustard in a small bowl. Set aside.

Once mushrooms are fork-tender, transfer to a cutting board. Cut mushrooms into 1/2-inch slices. Transfer back to the same baking sheet. Sprinkle cheese over mushrooms. Roast in the middle of the oven until cheese melts, 3-4 min. Remove from the oven and set mushrooms aside to cool slightly, 5 min. Meanwhile, arrange buns directly on the top rack of the oven, cut-side up. Toast until buns are golden-brown, 3-4 min. (TIP: Keep an eye on buns so they don't burn!)

Whisk together vinegar, remaining mustard, 1/2 tsp sugar and 2 tbsp oil (dbl both for 4 ppl) in a large bowl. Add spinach, then toss to combine. Season with salt and pepper, to taste.

Spread mustard mayo over buns. Stack mozza portobellos, crispy shallots, sliced tomatoes and some dressed spinach on bottom buns. Close with top buns. Add salad topping mix and chopped tomatoes to the bowl with remaining dressed spinach. Toss to combine. Divide burgers and salad between plates.