Mix the ground beef with half of the minced garlic, dried thyme, salt, and pepper in a bowl until just combined, then form into four equal patties.
Heat one tablespoon of oil in a skillet over medium-high heat. Place the beef patties in the pan and sear until browned on each side, about 3-4 minutes per side. Remove and set aside.
Add the remaining oil to the skillet. Sauté the sliced onions until they turn golden brown and start to caramelize, about 8-10 minutes. Add the remaining minced garlic during the last minute and cook until fragrant.
Pour in the balsamic vinegar to deglaze the pan, scraping up any browned bits from the bottom. Cook for 1-2 minutes until the vinegar reduces slightly and the aroma intensifies.
Return the seared beef patties to the skillet, placing them atop the caramelized onions. Pour in the beef broth, ensuring the patties are partially submerged.
Bring the mixture to a gentle simmer, then reduce the heat to low. Cover the skillet and let the patties cook through for about 15 minutes, until they are tender and the gravy has thickened slightly.
In a small bowl, whisk the cornstarch with a few tablespoons of cold water to create a slurry. Stir this into the simmering gravy to thicken it further, cooking for an additional 2-3 minutes until glossy and thick.
Taste the gravy and adjust seasoning with salt and pepper as needed. The sauce should be rich, glossy, and flavorful with a hint of tang from the balsamic.
Serve the Salisbury steak patties hot, smothered in the savory gravy and caramelized onions. Pair with mashed potatoes or your favorite sides for a hearty, comforting meal.
