Preheat oven to 350 F.
On a cutting board, peel the onion and dice it. Peel the garlic and mince. Trim the ends of the carrot and celery stalk and dice them into small cubes. Dice the tomatoes and set them aside.
Heat the olive oil in a sauté pan over medium high heat. Add the onion and garlic to the pan and cook 2 minutes, stirring frequently. Add the carrot and celery and sauté for 6-8 minutes or until starting to soften and brown. Turn off the heat.
Place the ground beef into a large bowl. Remove the casing from the raw sausage and add it to the beef, discarding the casing. Add the cooked vegetables to the bowl. Next, add the diced tomato, thyme, oregano, salt, black pepper, egg, and bread crumbs. Mix well to incorporate all the ingredients.
Line a loaf pan with parchment paper, trimming the paper if it over hangs too much. Fill the loaf pan with the meat and vegetable mixture.
To make the glaze, whisk together the honey, Dijon mustard, vinegar, melted butter, and black pepper in a small bowl. Remove the stem from the thyme and add it to the bowl. Season the glaze with salt, whisk again and taste. Add more salt as necessary.
Using a pastry brush, brush the honey mustard glaze on top of the meatloaf, reserving the leftover glaze for half way through cooking.
Put the meatloaf into the oven to cook for 30 minutes. Remove the meatloaf from the oven and brush the rest of the glaze on top of it before returning it to the oven for another 20-25 minutes.
Remove the meatloaf from the oven and check the temperature to make certain it reads 145F. Allow it to cool 5-10 minutes before slicing and serving.