1Slice yellow onions into rainbows and sauté in olive oil on low for 4-4.5 hours, adding butter or water as needed, and adding Worcestershire sauce and sugar in the last 20 minutes.
2Season chuck roast with salt, pepper, onion salt, minced garlic, rosemary, and thyme; cook in a slow cooker with chicken broth for 9-10 hours on low, then chill to remove hardened fat.
3Melt butter and sauté rosemary, thyme, aglio olio seasoning, minced garlic, and dry minced onion until fragrant.
4Add beef fat and flour to the pan and cook for 1-2 minutes.
5Gradually stir in broth and heavy cream, then mix in caramelized onions and pot roast.
6Stir in frozen mixed vegetables and adjust seasoning to taste.
7Top with puff pastry, brush with egg wash, and bake at 375°F for 35-45 minutes.
8Garnish with parsley and flaky salt before serving.