Heat the broiler, if using to char the poblanos.

• Cut a small corner from the steak packaging and drain off any excess liquid. Transfer to a plate; pat dry with a paper towel. Season generously with salt and pepper and the coriander.

In a large frying pan over medium-high heat, warm 1 to 2 teaspoons oil until hot but not smoking. Working in batches if needed, add the steak and cook, stirring occasionally, until browned but not yet cooked through, 2 to 4 minutes. Transfer to a plate. Add more oil between batches if needed. Do not clean the pan.
While the steak cooks, prepare the onion and beans.

• Peel and coarsely chop enough onion to measure 1 cup [2 cups].
• Rinse the pinto beans.

In the same pan used for the steak, if dry, add 1 to 2 tablespoons oil. Warm over medium-high heat until hot but not smoking. Add the onion, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Stir in the beans and tomatoes and cook until warmed through, 1 to 2 minutes. Add the steak and any accumulated juices and continue cooking, stirring occasionally, until cooked through, 2 to 3 minutes for medium. Remove from the heat and season to taste with salt and pepper.
While the steak-bean filling cooks, prepare the poblanos.

• Coarsely chop the cilantro for garnish.

On the stovetop directly over a flame, or on a sheet pan under the broiler, roast the poblanos, turning frequently, until blistered and charred all over, 5 to 7 minutes. Transfer to a cutting board to cool slightly. Cut the poblanos in half lengthwise and remove the ribs and seeds. Stuff the poblanos with the steak-bean filling. Wash your hands after handling.

Spread the romesco onto individual plates. Top with the chiles rellenos, garnish with the cilantro, and serve.