Preheat the oven to 425°F. Line a large baking sheet with parchment paper.
Place the broccoli, red onion, and red bell pepper on the prepared baking sheet. Drizzle with 1 tablespoon of olive oil and sprinkle with crushed red pepper flakes. Toss until evenly coated, then spread into an even layer.
Roast until the broccoli is bright green and browned in spots and the red bell pepper is slightly tender, about 18 to 20 minutes. Remove from the oven and let cool slightly, about 5 minutes.
Meanwhile, in a small bowl, whisk together the lemon zest, lemon juice, garlic, dry mustard, kosher salt, and black pepper. While whisking continuously, slowly drizzle in the remaining 3 tablespoons of olive oil until the mixture is emulsified, about 2 to 3 minutes.
Transfer the roasted vegetables to a large bowl. Pour in the lemon dressing and toss well to coat.
Sprinkle with Parmesan cheese and sunflower seeds before serving. Season with additional kosher salt to taste.