For vinaigrette, in a screw-top jar combine oil, lemon juice, basil, garlic, mustard, salt, and pepper. Cover and shake well. Set aside.
In a large saucepan cook green beans, covered, in a small amount of boiling water for 4 minutes. Add carrots and cook, covered, for 8 minutes more or until vegetables are crisp-tender. Drain and rinse with cold water. Combine beans, carrots, sweet peppers, mushrooms, and tomatoes in a bag set in a large bowl. Pour vinaigrette over vegetables.
Seal bag and refrigerate for 4 to 24 hours, turning bag occasionally. To serve, bring to room temperature. Drain vegetables; arrange on a platter. Makes 8 to 10 servings.