Preheat oven to 350°F.
For the tomato jam, arrange the tomatoes, garlic, thyme on a baking sheet and season with salt and pepper. Drizzle with olive oil and toss so everything is evenly coated. Season with salt and pepper.
Roast for 30 minutes. Allow to cool and roughly chop everything together. Reserve.
For the sammies, reheat the braised beef with the onions in some of the braising liquid. Stir in Dijon mustard and season with salt and pepper (filling should be moist but not too wet).
Assemble the sandwiches with a layer of meat, a thin layer of tomato jam, a layer of caramelized onions, some arugula and a few slices of cheese.
Close sandwiches and gently press down. Butter the outsides of the bread and griddle the sandwiches in a pan until toasted. Finish in oven for 3-5 minutes, until warmed through and the cheese is melted, pressing them down occasionally (or, put a light weight on it to keep them weighted).
Cut in half with a serrated knife and serve immediately!