Make Onions: In a large skillet, melt butter over medium-high heat. Add onions, cook for 2-3 minutes, just until beginning to soften. Lower heat to medium. To the skillet add garlic, thyme, vinegar, sugar, salt and pepper. Mix well. Cook over medium heat until golden and sweet about 12-15 minutes. Remove to a bowl and set aside. (The longer they cook the more intense the caramelization and flavor)
Preheat oven to 375°F. Line a large baking sheet with parchment or a silicone baking mat. Set aside.
Assemble Crescent Ring: Unroll crescent rolls, separating each triangle. Arrange in a sunburst pattern on prepared baking sheet, with pointed ends of the triangles facing outward. The bases should overlap.
Arrange 6 slices provolone over the widest portion base of the rolls. In a small bowl, whisk together cream cheese, mayonnaise and horseradish. Spread evenly over cheese.
Over the cream cheese, arrange roast beef, caramelized onions and final 6 slices of cheese forming a ring at the base. Fold triangle tips over filling and tuck under base to secure.
Brush on all sides with beaten egg wash and sprinkle with Parmesan cheese. Bake for 22-25 minutes or until golden and cheese has melted.
Make Au Jus: Meanwhile, in the same skillet used for onions, add 1 Tbsp butter and garlic. Cook for 1 minute to soften. Add stock, Worcestershire sauce and thyme. Bring to a gentle simmer, then lower the heat and let cook gently while crescent ring bakes. (You can make this up to 2 days in advance and reheat.)
Serve: After baking, rest on the counter for 3-5 minutes.
Slice into desired size pieces and serve au jus on the side for dipping.