Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Core, deseed, and thinly slice bell pepper. Halve, peel, and thinly slice onion. Very thinly slice mozzarella.

Toss bell pepper on a baking sheet with olive oil, salt, and pepper. Roast until softened and lightly charred, 18-20 minutes.

Meanwhile, heat a drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add onion; cook, stirring occasionally, until softened, 8-10 minutes. Stir in vinegar and 1 tsp sugar (2 tsp for 4 servings). Cook, stirring, until caramelized and jammy, 2-3 minutes. Season with salt and pepper. Turn off heat; transfer to a small bowl. Wash out pan.

While veggies cook, in a small pot, combine marinara sauce, stock concentrate, 3/4 cup water (11/2 cups for 4 servings), 1/2 tsp sugar (1 tsp for 4), and a big pinch of salt. Bring to a boil over medium-high heat. Once boiling, reduce heat to medium low and simmer for 4-5 minutes. Stir in cream cheese until melted. Cover and continue to simmer until ready to serve.

Heat pan used for onion over medium heat. While pan heats, place 3 TBSP butter (5 TBSP for 4 servings) in a small microwave-safe bowl; microwave until just softened, 10-20 seconds. Spread one side of each slice of bread with softened butter. Place buttered sides down on a work surface. Top half the slices with half the mozzarella, all the veggies, and remaining mozzarella. Complete sandwiches with remaining slices of bread, buttered sides up; press firmly.

Place sandwiches in preheated pan; use a spatula to press down. Cook until bread is golden brown and cheese is melted, 3-4 minutes per side. Transfer to a cutting board and slice in half on a diagonal; divide between plates. Season soup generously with salt and pepper; divide between bowls and serve on the side. TIP: Dip panini into soup, if you like!