Peel and halve onion. Slice halves into thin strips (julienne). Cut broccoli florets into bite-sized pieces.

Place a large pot over high heat with 2 Tbsp. olive oil. Add onions and cook undisturbed for 3 minutes, or until very dark brown. Stir onions and season with 1/2 tsp. salt and a large pinch of pepper. Reduce heat to medium, cover, and cook for 4 minutes.

While onions caramelize, place broccoli on prepared baking sheet. Drizzle with 2 tsp. olive oil, grainy mustard, a generous pinch of salt and pepper, and toss to evenly coat. Spread broccoli into a single layer and roast for 8 minutes, or until lightly charred. Move broccoli to a plate, replace foil, and turn broiler on high.

Uncover and increase heat to high. Add marsala wine and cook until almost completely evaporated, about 1-2 minutes. Add 3 1/2 cups water and demi-glace base and bring to a boil. Reduce to a simmer and cook 8-10 minutes, or until soup has reduced and slightly thickened.

Halve baguettes, place on baking sheet cut-side up, and drizzle with 1 tsp. olive oil. Place baking sheet under broiler until bread becomes golden brown, about 1-2 minutes. Rub garlic clove on cut sides of baguettes. Remove top halves to a plate. Add roast beef to bottom halves of baguettes and top with Swiss cheese. Place under broiler for 1-2 minutes, or until cheese is bubbly and lightly browned.

Divide broccoli between two plates. Place bread tops on each sandwich and serve next to broccoli. Ladle soup into two bowls and serve next to sandwiches. We recommend dipping your sandwich into the soup as you eat!