Process egg yolk, garlic, fish sauce, salt, and 1 tablespoon of the water in a food processor until smooth, about 2 minutes. With processor running, slowly pour canola oil through food chute until mixture is blended and thick, about 1 minute. Stir in remaining 1 tablespoon water to thin out mayonnaise, if needed. Cover and refrigerate until ready to serve. Prepare the Fish Sauce-Caramel Glaze:

Place sugar and 2 tablespoons of the water in a small saucepan. Cook, undisturbed, over medium-high until sugar mixture turns an amber color, 3 to 5 minutes. Stir in fish sauce, red pepper, and remaining 3 ⅓ tablespoons of the water. (Mixture will seize but that’s okay.) Bring mixture to a boil over medium-high, whisking or stirring to break up sugar. (To make sure it is a glaze consistency, dip a metal spoon into sauce; carefully run your finger on back of spoon to make a line in the sauce. If line holds on the spoon, your glaze is done. If line is runny, continue to boil for a few more minutes.) Remove from heat, and let cool, about 1 hour. Whisk in lime juice. Prepare the Bánh Mi Pickles:

Place radish and carrot in a medium heatproof bowl. Bring vinegar, water, sugar, and salt to a boil over high, stirring until sugar is dissolved, about 2 minutes. Pour hot vinegar mixture over radish-carrot mixture, and let cool, about 1 hour. Cover and refrigerate until ready to use. Prepare the Sticky Fried Catfish:

Whisk together breadcrumbs, cornmeal, flour, and salt in a large bowl; set aside. In a shallow bowl or pie plate, whisk together milk and eggs. Working in batches, dust catfish fillets with additional flour, and dredge in milk mixture, coating mixture evenly; dredge in breadcrumb mixture. Place dredged catfish on a plate or tray, and let stand for 5 minutes.

Pour oil into a large Dutch oven to a depth of 2 inches; heat over medium-high to 360°F. Fry fish, in 3 batches, until golden brown, about 3 minutes per side. Transfer fish to a paper towel-lined baking pan.

Toast bread loaves, and cut each loaf in half lengthwise. Spread Bơ on both sides of bread loaves. Brush both sides of Sticky Fried Catfish with Fish Sauce-Caramel Glaze. Place catfish on prepared bread loaves. Top evenly with lettuce, tomatoes, Banh Mi Pickles, cilantro, and a couple of dashes of hot sauce (if using).