Preheat oven to 400°F. Heat 1 tablespoon of the oil in a large nonstick skillet over medium. Add potatoes; cover and cook, stirring occasionally, until lightly browned, about 7 minutes.
Whisk together eggs, milk, salt, and 1 teaspoon of the mustard in a medium bowl. Set aside.
Add broccoli and garlic to potatoes in skillet; cover and cook, stirring occasionally, until broccoli is bright green and tender, about 3 more minutes. Stir in pepper strips, olives, and dill. Pour egg mixture into skillet, and increase heat to medium-high; cook, without stirring, until egg mixture begins to set around edges, 1 to 2 minutes. Sprinkle with feta, and bake in preheated oven until set, 8 to 10 minutes.
Meanwhile, combine vinegar, shallot, crushed red pepper, and remaining 1 teaspoon mustard in a small bowl. Whisk in remaining 1 tablespoon oil until dressing is thoroughly combined. Drizzle frittata with dressing, and garnish with chopped fresh dill. Serve immediately.