In a medium saucepan over medium-high heat, combine sour cream and heavy cream, bring to a simmer. Cook, whisking constantly and adjusting heat as needed to maintain a simmer, until sauce is thick and coats the back of a spoon, 4 to 8 minutes.
Reduce heat to medium-low, stir in mustard and thyme and simmer, whisking constantly, for 2 minutes more.
Season to taste with salt and pepper and serve warm.
