Preheat the oven to 425°F and line a baking sheet with parchment paper.
Toss the baby potatoes with olive oil, salt, and pepper. Spread them on one side of the baking sheet and roast for 10 minutes.
Add the green beans and cherry tomatoes to the baking sheet, tossing with olive oil, salt, and pepper. Roast everything for an additional 15 minutes, or until the potatoes are tender and golden.
While the vegetables roast, season the halibut with olive oil, salt, pepper, and a pinch of dried thyme. Place on a separate baking sheet and roast for 12-15 minutes, or until the fish flakes easily with a fork.
In a small bowl, whisk together olive oil, Dijon mustard, minced shallot, lemon juice, red wine vinegar, salt, and pepper to make the dressing.
Assemble the salad by arranging mixed greens on a large platter. Top with roasted potatoes, green beans, cherry tomatoes, halibut fillets, hard-boiled eggs, and olives. Drizzle with the dressing and garnish with capers or fresh herbs if desired.
Serve immediately and enjoy!