Onion Sauce Recipe

Prepare onions

Peel and thinly slice enough yellow onions to yield about 3 cups sliced (roughly 2 medium onions).

Mince the garlic and finely chop the shallot. Set aside.

In a large sauté pan over medium-low heat, add 1 tablespoon olive oil and 2 tablespoons butter. When the butter foams but before it browns, add the sliced onions in an even layer.

Cook onions slowly, stirring every 3–5 minutes, adjusting heat as needed so they soften and gently caramelize without burning. This should take 20–25 minutes. If they begin to stick, add a splash (1–2 tbs) of stock or water to deglaze the pan and continue.

When the onions are soft and golden-brown, add the minced garlic and chopped shallot and cook for 1–2 minutes until fragrant, stirring constantly so the garlic does not brown.

Push the softened onion mixture to one side of the pan. Add the remaining 1 tablespoon butter to the cleared space and melt over medium heat.

Sprinkle 1 tablespoon all-purpose flour into the melted butter and stir to form a paste (roux). Cook the roux for 1 minute to remove the raw flour taste, stirring constantly and incorporating it into the onions.

If using wine, pour 1 tablespoon dry white wine into the pan to deglaze, scraping up any browned bits. Let it reduce for 30–45 seconds.

Slowly whisk in 2 cups low-sodium chicken or vegetable stock, adding a little at a time to avoid lumps. Bring the mixture to a gentle simmer.

Add 1 teaspoon Dijon mustard, 1 teaspoon soy sauce (or tamari), 1 teaspoon freshly ground black pepper, 1 teaspoon kosher salt, and 1 teaspoon fresh thyme leaves. Stir to combine.

Let the sauce simmer gently for 8–10 minutes, stirring occasionally, until slightly thickened and flavors meld. Taste and adjust seasoning as needed.

Stir in 1 tablespoon heavy cream for silkiness and 1 teaspoon apple cider vinegar or lemon juice to brighten the sauce. Simmer 1 more minute.

If you want an ultra-smooth sauce, transfer the pan contents to a blender or use an immersion blender and puree until smooth (use caution with hot liquids). Return to pan.

Off heat, stir in 1 tablespoon cold butter (for mounting) until glossy. Stir in chopped fresh parsley if using.

Adjust final seasoning — add more salt, pepper, or a splash more vinegar/lemon if needed. Keep warm until ready to serve.