Toast the pecans in a dry skillet over medium heat, shaking almost constantly, until fragrant. Place them in the container of a blender or small food processor along with the garlic and pulverize.
Place the ground nuts and garlic in a bowl and add the vinegar, salt and pepper; stir. Add the oil a bit at a time. The mixture will become creamy and quite thick. Taste to make sure the acid balance pleases you and add more vinegar if you like; if the taste is good but the mixture is too think, add warm water, a teaspoon at a time. This is best made fresh but will keep, refrigerated, for a day or so; bring back to room temperature before using.