Melt the butter in a medium saucepan over medium-high heat. Add the onion and cook, stirring occasionally, until beginning to soften, 4 to 5 minutes.
Add the corn, half-and-half, ½ teaspoon salt, and ¼ teaspoon pepper. Simmer until the corn is tender and the half-and-half is slightly thickened, 5 to 8 minutes.
Meanwhile, heat broiler. Season the steak with blackening seasoning and ¼ teaspoon each salt and pepper. Place on a broilerproof rimmed baking sheet.
Broil steak for 3 to 4 minutes per side for medium-rare. Let rest for at least 5 minutes before slicing.
In a medium bowl, toss the greens and tomatoes with the oil and ¼ teaspoon each salt and pepper. Serve with the steak and corn.