Cook bacon in a large heavy-bottomed pot over medium heat until crisp, about 15 minutes. Transfer to a paper-towel-lined plate to drain, then coarsely chop.
Add onion to drippings in pot and cook until softened and golden in places, 8 to 10 minutes. (Add oil to prevent onion from burning, if needed.) Add garlic and cook until fragrant, about 2 minutes. Add butter, corn, and scallions and cook, stirring occasionally, until corn is crisp-tender, 10 to 12 minutes.
Return bacon to pot, add broth and milk, and bring to a boil. Reduce heat and simmer until most of liquid has evaporated, about 2 minutes. Remove from heat and stir in tomato and parsley. Season with salt and pepper. Serve warm or at room temperature.
Maque choux can be made up to 1 day ahead and stored in refrigerator; bring to room temperature before serving.