Combine 2 tablespoons olive oil, 3 tablespoons balsamic vinegar, orange juice, 2 teaspoons minced garlic, chili flakes, and 1/4 teaspoon each salt and pepper in a bowl and whisk. Place steak and mushrooms in a glass baking dish or sealable plastic bag and pour marinade on top. Marinate in the refrigerator for at least 1 or up to 4 hours.
Combine remaining 2 tablespoons olive oil, 3 tablespoons vinegar, 1 teaspoon minced garlic, water, mustard, and remaining 1/4 teaspoon each salt and pepper. Whisk until incorporated.
Place green beans in a steamer basket over a few inches of boiling water and steam until tender-crisp, about 3 minutes. Allow to cool in the refrigerator, or to cool quickly, drain and plunge in a bowl of ice water for 2 minutes. Drain and reserve.
Spray a grill or grill pan with nonstick cooking spray and heat over medium-high heat. Remove steak and mushrooms from marinade, and discard marinade. Grill steak until medium-rare and mushrooms until cooked through but still juicy, about 4 to 5 minutes per side for both steak and mushrooms. Remove to a cutting board and let rest about 10 minutes. Cut steak across the grain into 1/4-inch slices; cut mushrooms into 1/4-inch slices.
To assemble salad, toss steak slices, mushroom slices, spinach leaves, tomatoes, green beans and onions with dressing. Divide among 4 plates.