Tomato and roast beef salad

Ingredients

• 6 cups arugula
• 2 tablespoons olive oil
• 2 tablespoons red wine vinegar
• 1 teaspoon Dijon mustard
• 1 teaspoon prepared horseradish
• 6-8 slices roast beef, thinly sliced
• 1 cup cherry tomatoes, halved
• 1/4 cup crumbled blue cheese