Preheat the oven to 375°F. Peel the parsnips and cut into two or three segments, depending on their lengths. Then cut each piece lengthways into two or four. You want pieces roughly 2 inches long and ½-inch wide. Peel the onions and cut each into six wedges.
Place the parsnips and onions in a large mixing bowl and add ½ cup of the California Olive Ranch extra virgin olive oil, the thyme, rosemary, garlic, 1 teaspoon salt and some pepper. Mix well and spread out in a large roasting pan. Roast for 20 minutes.
While the parsnips are cooking, trim both ends of the sweet potatoes. Cut them (with their skins) widthways in half, then each half into six wedges. Add the potatoes to the pan with the parsnips and onion and stir well. Return to the oven to roast for a further 40 to 50 minutes.
When all the vegetables are cooked through and have taken on a golden color, stir in the halved tomatoes. Roast for 10 minutes more. Meanwhile, whisk together the lemon juice, capers, maple syrup, mustard, remaining 2 tablespoons California Olive Ranch extra virgin olive oil and ½ teaspoon salt.
Pour the dressing over the roasted vegetables as soon as you take them out of the oven. Stir well, then taste and adjust the seasoning. Scatter the sesame seeds over the vegetables if using and serve at the table in the roasting pan.