Set your oven's broiler to HIGH. Arrange the bread, cut side up, on a cookie sheet. Drizzle twith olive oil and pop the pan under the broiler. Toast until the bread is uniformly golden brown. Remove the pan from the oven and set aside.
Heat 1 tablespoon of olive oil in a heavy-bottomed skillet over medium-high heat. When it is shimmery, lay the onion slabs in the pan and let them fry 3-4 minutes on one side, undisturbed, or until you can ease a spatula underneath, lift it gently, and find it to be golden brown. Lay a flexible spatula over each slice, slide another one underneath, and gently turn each over to fry for another 3 to 4 minutes, or until golden brown on the second side. Transfer to a plate and tent gently with foil.
Return the pan to the burner, adjust the heat to low, and melt the butter. Sprinkle the flour over the melted butter and whisk it in. Toast for 1 minute, or until fragrant, whisking the whole time. Whisk in the beer until smooth, then the milk. Whisk in the dry mustard powder, Worcestershire sauce, paprika, and Dijon mustard until perfectly smooth. Stir in the grated cheese until melted, then remove from the heat and whisk in the egg yolk. Serve immediately with the toast, fried onions, fresh tomatoes, and bacon. The Welsh Rabbit is best served hot or warm.