Heat oil in heavy pan, add onions, turn heat to low and let onions cook and caramelize, stirring every few minute until onions are nicely brown (about 30-40 min., don't rush this step, be careful not to burn onions), add garlic & thyme; cook about 2 minute more. When onions have cooked 15 min., preheat oven to 375.
While onions are cooking put ground beef in a medium bowl, let come to room temp, mix together eggs, sour cream horseradish and worcestershire sauce.
When onions are done, add bread crumbs and onion mixture to meat, season with salt and pepper, then add egg/sour cream mixture. Use your hands to GENTLY combine until all ingredients are mixed. Handle meat as litttle as possible.
Spray broiler or roasting pan with non-stick spray, form meat into a loaf shape and place on pan.
Bake about 1 1/2 hours or until meatloaf is nicely browned on the outside and internal temperature is 155-160 degrees.
Let meatloaf rest 10-15 minutes before cutting; while it rests, stir together sour cream, may and horseradish to make a sauce.