Heat a grill or grill pan to medium-high. Clean and lightly oil hot grill. In a small bowl, stir together mustard and 2 tablespoons honey. Reserve 2 tablespoons honey mustard for tomorrow's lunch. Season chicken with salt and pepper. Grill until browned and cooked through, 12 to 15 minutes, flipping once. Brush with half the remaining honey mustard and cook 1 minute. Transfer chicken to a cutting board, tent with foil, and let rest 5 minutes. Cut 2 chicken breasts into thin slices and transfer to a serving platter. Reserve 2 chicken breasts for tomorrow's lunch.
Brush onions with vegetable oil, season with salt and pepper, and grill until browned and tender, 7 to 9 minutes, flipping once. Brush onions with remaining honey mustard and cook 1 minute. Reserve 1/2 cup onions for lunch and transfer remaining onions to serving platter.
In a medium bowl, whisk together 1/2 teaspoon honey, vinegar, and olive oil; season with salt and pepper. Add spinach and tomatoes and toss to combine. Serve salad alongside sliced chicken and onions.
Setting aside grilled chicken breasts and onion rounds means you can make sandwiches without having to fire up the grill. Making extra honey mustard tonight also adds fast flavor at lunch.