Bring a large pot of well-salted water to a boil. Add the beans and boil until bright green and tender, about 5 minutes. Use tongs to scoop out the beans and place in a colander to drain. Set aside.
Add the pearl onions to the boiling water and boil until tender, about 5 minutes. Drain well. When cool enough to handle, trim and peel the onions.
Cook the bacon in a skillet over medium heat, stirring occasionally, until it is very crisp, about 10 minutes. Using a slotted spoon, transfer the bacon to a plate lined with paper towels. Measure how much bacon fat is in the pan; there should be about 2 tablespoons.
Return the bacon fat to the skillet over medium-low heat and add enough olive oil to equal 4 tablespoons total fat. Add the mustard and cider vinegar and stir to combine well. Season to taste with salt and pepper.
Toss the beans and the onions with the hot dressing; taste and adjust the seasonings.
Transfer the vegetables to a serving dish and top with the crisp bacon.