Beurre Manié delivers a smooth, thicker sauce, but there are substitutes. You can use a pre-cooked (or even store-bought) roux, or stir in a cornstarch or arrowroot slurry (1 Tablespoon of cornstarch or arrowroot with 1 Tablespoon of cold water). Add these to a simmering sauce and completely whisk or stir until dissolved.