Pat salmon fillets dry with paper towels. Season both sides with salt and pepper. Heat olive oil in a large skillet over medium-high heat. When shimmering, add salmon fillets presentation-side down (skin-side up if skin-on). Sear without moving for 3-4 minutes until the edges begin to turn opaque. Flip carefully and sear another 2-3 minutes until the fish is just opaque in the center. Remove salmon from the pan and let rest while making the sauce.

Sauté aromatics

In the same pan, reduce heat to medium. Add minced garlic and chopped shallot; sauté 1-2 minutes until translucent and fragrant, scraping up browned bits from the salmon. Stir in fresh thyme leaves.

Build the grape-red wine sauce

Increase heat to medium-high and pour in red wine to deglaze, scraping up all caramelized bits. Simmer until the wine reduces by about half and thickens slightly, about 4-5 minutes. Stir in grape juice, lemon juice, and balsamic vinegar. Let the mixture return to a gentle simmer.

Incorporate grapes and finish sauce

Add red grapes and simmer until they begin to soften and release juice, about 3-4 minutes. Reduce heat to low and whisk in butter, one tablespoon at a time, until the sauce is glossy and slightly thickened. Taste and adjust salt, pepper, or lemon juice as needed.

Plate and serve

Return salmon fillets to the pan briefly to warm through and coat with sauce, about 30-60 seconds. Spoon the sauce over each fillet and garnish with a few thyme sprigs if desired. Serve immediately, with extra sauce drizzled around the salmon.