Heat grapeseed oil in a heavy-bottomed saucepan over medium-high heat.

Add diced red onion and sauté until translucent, about 3-4 minutes, stirring frequently.

Deglaze the pan with Cabernet wine, scraping up any browned bits from the bottom.

Add beef stock or demi-glace, chopped sun-dried tomatoes, rosemary sprig, and parsley sprigs.

Reduce the sauce by approximately 40%, which should take around 15-20 minutes.

Remove from heat and strain the sauce through a chinois or cheesecloth into a bowl to remove solids.

Return the strained sauce to the pan.

Stir in room-temperature butter to enrich and finish the sauce.

Serve immediately, keep warm, or store refrigerated in a covered container.