Heat oven to 400°. Toss Brussels sprouts with olive oil, 1/4 teaspoon of the salt and 1/8 teaspoon of the pepper. Place on a baking sheet and roast at 400° for 20 to 25 minutes, flipping sprouts once.
Place salmon skin-side down on a different baking sheet lined with nonstick foil. Season with remaining 1/8 teaspoon each salt and pepper. Spread mustard on top. Add to 400° oven during last 15 minutes of roasting Brussels sprouts.
Meanwhile, cook orzo in lightly salted boiling water for 8 minutes. Drain, reserving 1 cup of the cooking liquid.
In a large sauté pan, cook bacon over medium heat for 8 minutes; remove with a slotted spoon. In same pan, add onion to bacon fat. Cook until softened, about 3 minutes. Stir in lemon juice, mustard and 1/2 cup of the reserved pasta water. Bring to a boil and simmer 1 to 2 minutes to thicken. Stir in orzo and bacon. Add more pasta water, if desired.