To thicken a sauce in a slow cooker, use gentle methods that don’t require removing the pot from heat for long and that won’t sharply break the emulsion or cause scorching. Common and reliable options

• Cornstarch or arrowroot slurry: Mix 1–2 tablespoons of cornstarch (or arrowroot) with equal parts cold water to form a smooth slurry. Stir the slurry into the sauce, then cook on high uncovered for 10–15 minutes until thickened. Repeat if needed, but avoid over-thickening. This is one of the simplest, pantry-friendly methods.
• Flour-based slurry (roux-style): For a heartier texture, whisk 1 tablespoon all-purpose flour into a small amount of cool liquid to make a smooth paste, then whisk into the hot sauce. Cook on high with the lid off for 10–15 minutes, whisking occasionally to prevent lumps.
• Reduce by simmering: If practical, remove the lid during the final 15–30 minutes and let the sauce reduce and thicken as excess liquid evaporates. This concentrates flavors but watch closely to avoid burning.
• Potato starch or tapioca starch: These starches can be used similarly to cornstarch. Mix with cold water to a slurry and add to the sauce, then cook until thickened.
• Purees and dairy enrichments: A small amount of dairy (heavy cream or yogurt) or a pureed vegetable (like a little mashed potato or cooked cauliflower) can thicken and add body, but add gradually and monitor flavor and texture. For dairy, heat gently to avoid curdling.
• Roasted or mashed vegetables: For a natural body, blend some cooked vegetables into the sauce to thicken and enrich the texture without adding starches.
• Thickening with a roux mid-cooking: In larger, richer sauces, you can prepare a small roux separately (equal parts fat and flour cooked briefly) and whisk it into the sauce near the end of cooking to achieve desired thickness.

Tips for best results

• Start with small amounts of thickener and add gradually to reach the desired consistency.
• If using thickeners, dissolve first in a small amount of cold liquid to avoid lumps, then whisk into the hot sauce.
• Avoid adding thickening agents directly to very hot liquid in a blender or immersion blender, which can cause splattering; instead, whisk in gradually.
• Always adjust seasonings after thickening, as some thickeners can dull or alter flavors slightly.

If you’d like, share the specific sauce type (e.g., tomato-based, cream-based, meat sauce) and the currently measured consistency, and I can tailor a precise thickening approach and amounts.