Seared Salmon with Summer Vegetables

Name: Seared Salmon with Summer Vegetables
Cook time: 1 h
Total time: 1 h
Cuisine: American
Ingredients: 3/4 cup unsalted butter (6 ounces), divided; 1 cup thinly sliced red onion; 3 garlic cloves (2 thinly sliced and 1 whole), divided; 1/4 pound shiitake mushrooms, stems removed and reserved, caps quartered, divided; 1 medium-size ripe tomato, coarsely chopped; 1 teaspoon whole black peppercorns; 1 bay leaf; 1/3 cup balsamic vinegar; 1/3 cup water; Salt; Freshly ground black pepper; 3 tablespoons olive oil, divided; 1 pound fresh spinach; 2 cups fresh corn kernels; 1 (1-pound) center-cut salmon fillet, sliced crosswise into 6 strips; 3 tablespoons finely chopped chives