
Preheat the oven to 400°F. Transfer the potatoes to a baking sheet. Drizzle with olive oil and season with kosher salt and pepper. Toss to coat and then spread out on the baking sheet. Roast the potatoes for 35 to 40 minutes until the potatoes are tender and browned.
Place a skillet over medium heat and add the butter and 1 tbsp olive oil. Add the finely diced shallot and sauté for 3 to 4 minutes until the shallots are softened. Add the minced garlic and sauté for another minute. Carefully add the broth to the skillet and bring to a simmer. Whisk in the Dijon and grainy mustards, ½ tsp kosher salt, and ¼ tsp pepper. Allow the broth to simmer for a few minutes until it reduces by about a third.
Add the cream to the reduced broth and bring it back to a simmer. In a small cup or bowl, combine the cornstarch and water to make a slurry. Whisk the slurry into the cream sauce and cook for another minute until the sauce comes to a boil and thickens. Add the lemon juice and check the seasoning.
To serve, arrange a layer of potatoes in a serving bowl and spoon over some of the sauce. Sprinkle with a little chopped dill. Repeat two or three times with the remaining potatoes, sauce, and fresh dill. Enjoy!