Preheat oven to 425°F.
In a large bowl, combine broccolini, baby Portabello mushrooms, asparagus, green beans, yellow squash, and red pepper.
Toss with olive oil, salt, garlic powder, and pepper.
Spread on foil-lined baking sheets.
Roast for 15 minutes until crispy and tender.
Let cool to room temperature, then chill.
Whisk dijon mustard, horseradish, and water to make the dressing.
Pour over chilled veggies and toss.