Preheat the oven to 400 degrees F.
In a small pot, combine the 1/4 cup plus 2 tablespoons reserved garlic-pepper oil and the lemon pepper seasoning. Heat over low heat until just warm. Remove from the heat and let sit for 10 minutes to allow the lemon pepper seasoning to infuse the oil.
Cut the large thick stems off the head of cauliflower. Break the head up into small florets. Slice the potatoes and parsnips in approximately 1-inch thick pieces. Place all the vegetables into a large bowl, add the half the seasoned oil, and toss to coat. Add the remaining oil and toss again. Transfer to 2 baking sheets and roast until lightly brown and tender, 35 to 40 minutes. If the vegetables start to brown too quickly, loosely cover with foil. Transfer to a serving bowl.