Preheat the oven to 400 degrees . Roast the chile and bell pepper and cut into 1/4-inch pieces.
In a medium saucepan, melt the butter over medium heat. Add the onion and cook, stirring, until deep golden brown, about 7 minutes; transfer to a blender or food processor. Reduce the heat to low, add the garlic and cook for 1 minute; transfer to the blender. Reserve the saucepan.
Blend the onion mixture with the corn kernels and masa harina. Pour in 1-1/2 cups milk and process until smooth. Press the mixture through a strainer to remove any tough corn hulls. Pour the soup into the reserved saucepan and bring to a boil, whisking frequently, about 2 minutes. Add the remaining 2 cups milk (a little less if you're using frozen corn) and chile and bell pepper pieces and return to a boil. Reduce the heat and simmer, stirring frequently, for 15 minutes.
Meanwhile, cut the tortillas in half, then cut crosswise into 1/4-inch strips. Spread in a single layer on a baking sheet lined with parchment paper, drizzle with oil and toss to coat. Bake, tossing every few minutes, until lightly browned, about 8 minutes.
Add more milk to the soup if it's too thick; season with salt. Ladle into bowls; top with tortilla strips and crabmeat, if using.