Preheat the oven to 400°F. Place the baby potatoes on a baking sheet. Drizzle with extra virgin olive oil and season with kosher salt and pepper. Toss to ensure they are well coated, then spread them out evenly on the baking sheet. Roast the potatoes for 35 to 40 minutes, or until they are tender and browned.
In a skillet over medium heat, add the butter along with 1 tablespoon of olive oil. Introduce the finely diced shallot and sauté for 3 to 4 minutes until softened. Then, add the minced garlic and sauté for an additional minute. Carefully pour the broth into the skillet and bring it to a simmer. Whisk in the Dijon and grainy mustards, along with ½ teaspoon of kosher salt and ¼ teaspoon of pepper. Allow the broth to simmer for a few minutes until it has reduced by about a third.
Incorporate the heavy cream into the reduced broth and bring it back to a simmer. In a small cup or bowl, mix the cornstarch with the water to form a slurry. Whisk the slurry into the cream sauce and continue cooking for another minute until the sauce boils and thickens. Add the lemon juice and adjust the seasoning as needed.
To serve, layer some of the roasted potatoes in a serving bowl and drizzle with a portion of the creamy sauce. Sprinkle with a bit of chopped dill. Repeat this process two or three more times with the remaining potatoes, sauce, and fresh dill. Enjoy your creamy mustard roasted potatoes!
