Heat oven to 425°F Coat 9x13 pan with oil. Place English muffin pieces on a rimmed baking sheet and bake until golden brown, 12 to 15 minutes. Transfer to prepared baking dish. Reduce heat to 350°F.
Heat the oil in a large skillet over medium heat. Add the leek and cook, stirring occasionally, for 4 minutes. Increase the heat to medium-high, add the bacon and cook, stirring occasionally, until beginning to brown, 4-5 minutes more. Spoon over the English muffin pieces.
In a large bowl, whisk the whole eggsmilk, dry mustard, chives, 1-1/2 C cream, and 1/2 tsp cayenne, 1/2 tsp salt, and pepper. Pour over the leek-bacon mixture and bake until casserole is puffed, golden brown and just set, 40 to 45 minutes.
Ten minutes before the casserole is finished, make the hollandaise sauce. Melt butter in microwave. Whisk together, in a double boiler, the egg yolks, lemon juice, Dijon mustard, remaining 1/2 C cream, 1/4 tsp cayenne, and 1/2 tsp salt. Heat mixture, over simmering water, whisking constantly, until the mixture is thick enough to coat the back of a spoon, 2-3 minutes. Reduce heat to low. Still whisking constantly, gradually add the melted butter. Sprinkle the casserole with additional chives and serve with hollandaise.