Meanwhile, whisk to combine the half-and-half, sugar, eggs, and evaporated milk in a large bowl. Add banana mixture and bread cubes, mixing well to saturate bread with wet ingredients.
Transfer mixture to the prepared baking pan. Bake at 350 F for 20 minutes.
To prepare streusel, combine flour, walnuts, and next 3 ingredients (through ⅛ teaspoon salt), mixing with a fork; mix in oil until mixture is crumbly. Remove strata from oven; sprinkle evenly with streusel. Bake an additional 30 minutes or until golden and set.