Preheat oven to 350°F. Microwave bell peppers on HIGH 2 minutes to soften slightly. Cut bell peppers in half through the stem. Remove seeds and white membranes using a serrated tomato corer or melon baller, leaving stem intact. Discard seeds and membranes.

Heat canola oil in a large skillet over medium-high. Add sausage to skillet; cook 4 minutes, stirring to break into small pieces. Add onion to skillet; cook, stirring occasionally, until sausage is cooked and onion is tender, about 3 minutes. Remove sausage mixture from pan. Add cornmeal to skillet; cook 1 minute, stirring constantly. Add broth, milk, and 3/4 teaspoon of the salt to skillet; bring to a boil. Cover, reduce heat to medium, and cook, stirring occasionally, until liquid is absorbed, about 20 minutes. Stir in parsley, sausage mixture, and half of the cheese. Divide mixture evenly among bell pepper halves. Place side by side on a baking sheet. Sprinkle with remaining mozzarella cheese.

Bake in preheated oven until cheese is browned and peppers are tender, about 30 minutes.

Whisk together olive oil, vinegar, black pepper, and remaining 1/4 teaspoon salt in a medium bowl. Add tomatoes and basil; toss to coat. Serve over peppers.