Grainy mustard mixed with bacon and diced vegetables tops steamed artichokes. The mustard and bacon dressing is best when made and served immediately. The artichokes, however, can be prepared in advance.
Make an ice-water bath: Fill a bowl with ice and water; set aside. Trim artichokes; snap off tough outer leaves. Cut in half, and cut off top quarter of artichoke. Snip remaining leaf tips with scissors. Trim bottom of stem; using a vegetable peeler, peel off tough outer skin. Spread leaves to gain easier access to choke; scoop out choke with a melon baller. Squeeze some lemon juice onto heart; squeeze more lemon juice into the ice bath; add lemon halves and artichoke halves to ice-water bath while preparing the rest.
Fill a saucepan large enough to accommodate all artichokes with 2 inches of water. Add salt, peppercorns, thyme, garlic, and 2 tablespoons olive oil; bring to a simmer. Add artichokes, stem end up; cover saucepan. Steam until tender, about 25 minutes. Leaves should pull off easily; heart should feel tender when pierced. Drain well. Cool to room temperature.
Meanwhile, in a large saute pan, cook bacon over medium-low heat until bacon is brown and crisp and fat is rendered. Using a slotted spoon, remove bacon from saute pan, and set aside.
Depending on the amount of bacon fat in the saute pan, add enough olive oil to make a total of 1/3 cup fat and oil. Add shallots, carrots, and celery, and cook until vegetables are soft and fragrant, about 4 minutes. Stir in grainy mustard, and season with salt and pepper. Return bacon to saute pan. Arrange artichokes on serving platter. Spoon hot mixture into the cavity of each artichoke. Serve immediately.