Preheat the oven to 425°F.
Cook your pasta, drain, and set aside.
In a large oven proof skillet or dutch oven warm olive oil over medium low heat, then add the onions and cook them until caramelized and deep golden brown – about an hour – stirring occasionally.
To the onions add the garlic, worcestershire, and a pinch of salt. Cook for 2 minutes then remove to a bowl and set aside.
Add the butter to the same skillet. Once melted add the flour and mix together until dissolved, about 1 minute.
Add the milk and heavy cream, bring to a simmer over medium heat and cook until thickened slightly, about 5 minutes.
Take it off the heat and stir in the cheese until melted, followed by the caramelized onions.
Add the cooked pasta to the cheese sauce and stir to combine.
In a small bowl combine melted butter, breadcrumbs and thyme.
Sprinkle the breadcrumbs over the top of the mac and cheese in an even layer.
Bake until bubbly and golden brown on top, about 15-20 minutes.