Bring a large pot of water to a boil. Season the water with kosher salt and add the baby potatoes. Cook the potatoes just until they are fork-tender. They should be cooked through but not falling apart. Drain the potatoes and set aside to cool slightly.
For the radishes, trim and reserve the leaves. Cut the radishes in half and set aside. Pick through the leaves, reserving the large ones and discarding any small or damaged ones. Wash the leaves and allow to dry.
Preheat the oven to 425°F. Lightly grease a large sheet pan. Spread the cooked potatoes out evenly on the baking sheet. Using the bottom of a glass, or your palm, press down to smash the potato. Repeat with the remaining potatoes. You may need to use two baking sheets. Drizzle the smashed potatoes with olive oil and season with kosher salt and pepper.
Roast the potatoes for 20 minutes or until the bottoms begin to brown. Flip the smashed potatoes over and drizzle with more olive oil. Season with a bit more kosher salt and pepper. Continue roasting until the potatoes are crispy and well browned, another 30 to 40 minutes.
After flipping the potatoes, begin to roast the radishes. Transfer the radishes to a sheet pan and drizzle with olive oil. Season with kosher salt and pepper, tossing to coat. Spread the radishes out and roast for 25 to 30 minutes until they are tender when pierced and are beginning to brown.
Just before the radishes are finished roasting, toss the leaves with a little olive oil and kosher salt. In the last few minutes of roasting, scatter the leaves over the radishes. Roast for another 5 to 7 minutes until the radishes are tender and the leaves begin to crisp.
Prepare the vinaigrette while the potatoes and radishes are roasting. Add the bacon to a skillet and place over medium heat. Sauté the bacon until browned and crispy. Add the garlic and briefly sauté until fragrant.
Stir in the grainy mustard and honey and then carefully add the apple cider vinegar - it may splatter when added to the hot fat. Let simmer for another minute then add the kosher salt and pepper and remove from the heat. The vinaigrette can be made ahead of time and rewarmed before serving.
To serve, transfer half the smashed potatoes onto a serving platter. Top with half of the radishes and leaves and drizzle with half of the bacon vinaigrette. Repeat with the remaining potatoes, radishes, and vinaigrette. Enjoy!