How to Make It
Combine fish sauce and 1/3 cup onions in a shallow dish; add steak, turning to coat. Let stand at room temperature 1 hour, turning after 30 minutes.
Meanwhile, cut 1 bell pepper into 1-inch pieces; place in a saucepan with 1 cup water and 1/4 cup onions. Bring to a boil. Cook over medium heat 10 minutes or until tender. Drain and transfer to a food processor; process until smooth. Stir in 1 tablespoon oil and 1/4 teaspoon salt.
Slice remaining bell pepper. Heat a large cast-iron skillet over high heat. Add 1 tablespoon oil; swirl. Add sliced bell pepper, corn, and remaining 3 tablespoons onions. Cook 10 minutes or until peppers are wilted and corn is lightly charred, stirring occasionally. Stir in black pepper and 1/4 teaspoon salt. Remove from pan; cover and keep warm.
Remove steak from marinade; discard marinade. Return skillet to high heat. Add steak to pan; cook 3 minutes on each side or until glazed outside. Remove from pan; let stand 5 minutes. Cut diagonally into slices. Serve with corn mixture and pepper puree. Sprinkle with thyme, if desired.