Heat oil and butter in a large heavy pot over medium heat. Add bacon and cook, stirring often, until fat is rendered and bacon is crisp, 5-8 minutes. Add leeks and season with salt. Increase heat to medium-high and cook, stirring often, until leeks begin to brown, 5-8 minutes. Add cream, tarragon, pernod and 1/2 cup water. Bring to a boil, reduce heat, and simmer, stirring occasionally, until sauce is thickened and coats the back of a spoon, 5-8 minutes.