In a large saucepan, melt butter with the olive oil on medium-high until hot. Add the garlic and stir for 30 seconds. Add onion and cabbage, along with about 1 teaspoon of kosher salt and a few grindings of fresh pepper. Cook, stirring occasionally, about 20 minutes until the cabbage and onions have turned a golden color and lost all of their liquid. Stir in raisins in cognac, vinegar, red pepper flakes (start with half and add more if you like), and thyme leaves. Cook for about 2 to 3 minutes, until the cognac’s alcohol has cooked off. Allow to cool. Taste and add salt and pepper, if desired.