Step 1

Preheat oven to 350°. Heat 3 Tbsp. oil in a large skillet over medium. Add onions, garlic, bay leaves, red pepper flakes, 1 thyme sprig, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt and cook, stirring occasionally, until onions are golden and jammy, 15–20 minutes.

Step 2

Meanwhile, stir cream cheese, mayonnaise, and sour cream in a large bowl to combine (a few lumps are okay).

Step 3

Remove bay leaves and thyme sprig from onions; discard. Transfer onions to cream cheese mixture. Add 3 cups Gruyère and season with pepper; stir to combine. Transfer dip to an 8" cast-iron skillet or 2-qt. ovenproof baking dish and spread into an even layer with a rubber spatula.

Step 4

Toss panko, remaining Gruyère, remaining 1½ tsp. oil, and remaining ¼ tsp. salt in a medium bowl to combine, then sprinkle over dip. Bake until heated through and top is lightly browned, 25–30 minutes.

Step 5

Heat broiler. Broil dip until bubbling and golden brown, about 3 minutes. Let cool 5 minutes.

Step 6

Scatter more thyme over dip and serve with baguette, pears, and endive alongside for dipping.Do ahead: Dip (without panko mixture) can be made 3 days ahead. Cover and chill. Top with panko mixture just before baking.