Heat the remaining 1 1/2 teaspoons oil in a large skillet over medium-high heat. Add bell pepper and cook, stirring occasionally, until softened, about 5 minutes. Add the sliced white and light green scallions, garlic, butter and pepper; cook, stirring occasionally, until fragrant and softened, about 1 minute. Add flour and cook, stirring constantly, for 1 minute. Add broth and half-and-half; bring to a simmer, whisking constantly. Add the seasoned shrimp; reduce heat to medium and cook until the shrimp are just cooked through and opaque and the sauce has thickened slightly, 2 to 3 minutes, turning the shrimp over halfway through. Remove from heat.