"If there were ever a recipe to tweak to your own tastes, it's this one. You can adjust the amounts of garlic and anchovy, as well as the proportion of olive oil to butter. You can also control how long you cook the mixture before it's presented. I hear from my friends in Northern Italy that they use twice as much anchovy and garlic as I did. Serve in a small chafing dish or fondue pot with crusty bread, roasted carrots, blanched asparagus, endive, red peppers, and/or roasted potatoes."
Grind garlic and salt together with a mortar and pestle. Add anchovies; grind with mortar and pestle until a coarse paste forms.
Heat olive oil in a skillet over medium heat. Whisk butter into oil until butter melts and foams, about 30 seconds. Reduce heat to low and stir garlic-anchovy mixture into oil mixture; cook and stir until flavors blend and mixture is hot, about 5 minutes. Remove from heat and stir red wine vinegar and red pepper flakes into oil mixture.