¼ teaspoon salt
Thin avocado slices and sprigs of fresh flat-leaf parsley for garnish
To make the sauce, whisk together all the ingredients in a small saucepan until smooth. Set aside.
To make the patties, heat 2 tablespoons of the oil in a large skillet over medium-high heat. Add the carrots, onion, and celery; cook, stirring occasionally, until tender, about 8 minutes. Add the garlic; stir constantly for about 1 minute. Remove from the heat and stir in the remaining ingredients, except the garnishes. Let cool.
For each patty, gently squeeze about ¼ cup of the mixture in your hand to form a ball and then flatten into a 3-inch round, about 3/8-inch thick. Place the patties on a plate as they are prepared. Wash the skillet.
Heat the remaining 2 tablespoons of oil in the skillet over medium heat. Cook the patties for about 5 minutes on each side or until lightly browned.
To serve, arrange 3 patties on each plate; garnish with the avocado slices and parsley sprigs.