For the cream cheese: In a medium mixing bowl, combine the cream cheese, chervil, salt and lemon zest, mixing well. Adjust seasoning to your preference. Set aside.
For the onions: Place the butter, onions and a pinch of salt and pepper in a heavy duty pot over medium to low heat, stirring frequently with a wooden spoon as the onions tend to stick. Cook until amber brown, about 30 minutes, adding water as needed to pick up the browned bits on the bottom of the pan. Remove from the heat.
Place the cucumbers and dill in a glass bowl. In a small saucepan combine 1/2 cup water, the vinegar, sugar and salt. Bring to a simmer, then remove from the heat and pour over the cucumbers. Let cool to room temperature, 15 to 20 minutes. Once cool, store in the fridge in an airtight container.
For the sandwich build: Spread a healthy amount of the prepared cream cheese on the toasted bagels. Top each with 4 to 5 slices of lovely lox, the caramelized onions and a few of the marinated cucumbers, and serve the rest of the cucumbers on the side¿and you are killin' it!