Peel and halve red onion. Slice halves into thin strips like sticks (julienne). Stem, seed, and slice red bell pepper into thin 1/4" strips. Trim zucchini ends, halve lengthwise, and cut into 1/2” half-moons. Using a spoon, scoop out black undersides (gills) of portobello mushrooms. Cut mushrooms into 1/4" strips. Hold romaine head at root end and chop coarsely.

Place red bell pepper, zucchini, portobello and half the red onions on prepared baking sheet. Drizzle with 1 Tbsp. olive oil and season with 1/2 tsp. salt and a pinch of pepper. Roast 12-15 minutes, or until slightly caramelized. Remove from oven and let cool slightly.

While vegetables are roasting, combine balsamic, Dijon, and 2 tsp. olive oil in a small bowl. Mix together and season with a pinch of salt and pepper. Set aside.

Evenly spread pesto on two slices of sourdough. Place one slice of cheese on top of pesto. Portion roasted veggies onto two slices of bread and top with other bread slice.

Heat 1/2 tsp. olive oil in a medium pan over medium heat. Place sandwich in hot pan. Using an additional small or medium pan, press down on sandwich and cook 3 minutes, or until browned. Lift sandwich with spatula, add additional 1/2 tsp. olive oil to pan, and flip sandwich onto second side. Cook for an additional 3 minutes, or until browned. Repeat process with second sandwich.

Toss romaine, arugula, and remaining red onion together. Divide between two plates. Slice sandwiches in half and nestle against salad. Drizzle dressing over salad and enjoy!