Preheat a grill or grill pan over medium-high. In a skillet, cook the sausage over medium, stirring occasionally and breaking up with a wooden spoon, until browned, about 8 minutes. Stir in the garlic and transfer to a large bowl to cool. Stir in the fire-roasted tomatoes and cheese; season with salt and pepper.
Brush the outsides of the halved poblanos with 1 tbsp. oil; season all over. Spoon the sausage mixture into the poblano halves and transfer to the grill. Cook, covered, until the poblanos are tender and the filling is warm, 10 to 15 minutes.
Meanwhile, in a medium bowl, toss the plum tomatoes, bell pepper, red onion, cilantro, vinegar and the remaining 3 tbsp. oil; season the salsa. Top the poblanos with the sour cream and salsa.