1. Cut florets from broccoli. Trim florets into 1-inch pieces; trim stems, peel and cut into 1-inch pieces.
2. Bring a large pot of salted water to a boil over high heat. Add broccoli and cook until tender but not mushy, 3 to 4 minutes. Drain in a colander under cold running water. Pat broccoli dry with paper towels.
3. In a large bowl, combine mustard, vinegar, salt and pepper. Whisk together until salt is dissolved and then slowly add oil, whisking constantly until blended and thickened.
4. Just before serving, add broccoli and peppers to bowl with vinaigrette. Toss until vegetables are thoroughly coated with dressing. Season with salt and pepper. Serve at room temperature, or cover and refrigerate to serve cold.