Preheat the oven to 400°F
Season the cauliflower florets with salt and pepper and toss with 1 tablespoon olive oil on a baking sheet. Roast until golden and tender, about 15 minutes.
Heat a large skillet or grill pan to high. Pat the steaks dry and season with salt and pepper. Quickly sear the steak for 2-3 minutes per side to form a crust. Transfer to a baking sheet and roast until the steak reaches an internal temperature of 140℉ then remove from the oven and rest for 3-5 minutes before slicing.
Add all the vinaigrette ingredients to a jar with a lid and shake until emulsified. Season to taste.
Add all chimichurri ingredients except the oil to a blender. Purée until smooth. With the motor running, drizzle in the oil until emulsified. Season to taste.
To serve, toss the greens, tomatoes and roasted cauliflower with the mustard dressing in a large serving bowl. Serve with cauliflower, grilled steak and chimichurri.