In a large skillet brown the turkey and season with salt.: As the turkey hits the hot pan you should hear a definite sizzle and notice the meat losing its raw pink color, developing small browned bits that carry deep savory flavor. Browning creates Maillard reactions which add complexity to the filling, so resist the urge to crowd the pan and instead let the meat sear in batches if needed. A common mistake is stirring too frequently, which prevents proper browning and yields a steamed, pale texture. Aim for even coloration with small golden flecks, and season lightly with kosher salt so the meat is seasoned from the start.
When the turkey is browned, add onion, garlic, black beans, cilantro, serrano pepper, diced tomatoes, and cumin. Mix well and simmer on low, covered for 5 minutes.: You will notice aromatics like onion and garlic soften and perfume the kitchen, while the canned black beans and diced tomato begin to release moisture that marries with the meat. Covering traps steam and helps flavors meld quickly; the short covered simmer softens the ingredients without breaking them down. Avoid overheating which can make the tomato separate and the mixture too soupy, instead keep a gentle low heat so flavors combine evenly.
Remove lid, add corn and simmer an additional 5 minutes or until all the liquid reduces.: With the lid off you will hear a quiet bubbling, and steam will evaporate so the filling firms up and concentrates in flavor. The frozen corn should warm through and add sweet pops of texture against the softer components. If you skip this reduction the filling can be watery and the peppers will end up soggy, so watch for the mixture to thicken and for a cohesive, scoopable consistency.
Preheat oven to 350°.: When you set the oven to 350°, you should feel the kitchen warm slightly as it comes up to temperature, which ensures even roasting of the pepper shells. This moderate heat softens the peppers without burning them, allowing the filling to finish cooking inside. A frequent oversight is placing the peppers into an oven that has not fully preheated, which can lengthen the bake time and affect texture. Wait until the oven registers the temperature before inserting the dish.
Cut peppers in half lengthwise, removing seeds and stem.: The act of halving each red bell pepper should reveal crisp membranes and shiny flesh; remove the seeds and stem so the pepper forms a stable boat for filling. Cleaning them thoroughly prevents bitter bits and ensures even roasting. A common error is leaving too many seeds or ragged edges which can cause an uneven bake and a less tidy presentation. Aim for smooth halves that sit flat in the baking dish.
Place peppers, cut side up in an oven-proof dish. Fill each pepper with 1/2 cup turkey mixture. Pour about 1/3 cup water or chicken broth on the bottom of the dish.: Arranging the pepper boats cut side up keeps the filling contained and allows steam from the liquid at the bottom to tenderize the peppers. When scooping the filling you will feel a cohesive mass that holds its shape; adjust so each pepper receives a consistent portion for even cooking. If you skip the broth at the bottom the peppers can dry out; conversely, too much liquid will steam rather than roast them, so keep it to a small amount like the suggested 1/3 cup.
Cover tight with foil. Bake 45-50 minutes, or until the peppers become soft.: With the dish covered, the trapped steam will gently soften the pepper flesh; you should be able to pierce the pepper with a fork and feel tender resistance but not collapse. The cooking time allows the filling to finish cooking through and for the pepper to develop a mellower, sweeter profile. A common pitfall is underbaking which leaves the pepper tough, or overbaking which makes it limp; check for a tender but intact pepper after about 45 minutes.
Remove foil, top each with 1 1/2 tbsp of cheese and bake uncovered an additional 5 minutes.: Once you pull the foil back you will see bubbling moisture on the surface; adding the shredded cheese and returning the dish briefly to the oven creates golden melted pockets that add creaminess. Five minutes is usually enough to melt and slightly brown the cheese without drying the filling. A mistake some people make is leaving the cheese on too long which can harden, so watch carefully for a melted, slightly glossy finish.
Top with scallions and serve with reduced-fat sour cream if desired (optional).: The finishing touch of fresh scallions adds bright color and a mild sharpness against the warm, rich filling. If you choose the optional reduced-fat sour cream it will add a cool, tangy contrast that complements the spice from the serrano. Avoid adding wet garnishes too early, which can make the peppers soggy; sprinkle the scallions at the last moment and serve immediately for the best texture.