01 - Position oven rack to center position and preheat to 375°F. Rinse bell peppers, slice 1/2 inch from tops, remove seeds and white membranes. Stand peppers upright in a 9x13-inch baking dish, brush interiors lightly with olive oil. Roast for 10 minutes until slightly softened. Remove from oven but keep in dish.
02 - Place diced potatoes in a large saucepan, cover with cold salted water by 1 inch. Bring to a rolling boil over high heat. Reduce to medium-high and cook until fork-tender, approximately 10-12 minutes. Drain thoroughly in a colander and set aside.
03 - While potatoes boil, heat 2 tablespoons olive oil in a large skillet over medium heat until shimmering. Add chopped red onion and cook for 2 minutes until translucent. Add minced garlic and continue sautéing for 1 minute until fragrant, being careful not to brown the garlic.
04 - Stir in diced zucchini and corn kernels. Cook for 3 minutes, stirring occasionally, until zucchini begins to soften but still retains slight crunch. Add drained black beans and toss gently to combine.
05 - Add cooked potatoes to the skillet. Sprinkle with Cajun seasoning, smoked paprika, dried thyme, 1 teaspoon salt, and 1/2 teaspoon black pepper. Stir gently to coat all ingredients evenly. Continue cooking for 2 minutes to meld flavors. Remove from heat.
06 - Using a potato masher or fork, lightly mash the potato mixture, leaving visible chunks for texture. Pour in milk and add half of shredded cheese (1/4 cup). Add half of chopped parsley (1 tablespoon). Fold together until cheese begins to melt and mixture holds together slightly.
07 - Spoon the filling generously into each pre-roasted bell pepper, packing lightly and mounding slightly above the rim. Distribute any remaining filling evenly among peppers. Sprinkle tops with remaining cheese (1/4 cup) and breadcrumbs (1/4 cup).
08 - Return baking dish to oven and bake for 25-30 minutes until cheese is melted and bubbly, breadcrumbs are golden brown, and peppers are completely tender when pierced with a knife. Remove from oven and let rest 5 minutes.
09 - Sprinkle remaining fresh parsley over the stuffed peppers. Transfer to serving plates using a spatula, being careful to preserve shape. Serve immediately while hot.