Boil sweet potatoes and pumpkin in a saucepan until just tender; drain and set aside.
Repeat with brussels sprouts and set aside. (If using frozen vegetables, skip this step and just add to skillet after browning the leeks and garlic in the next steps.)
Heat two tablespoons olive oil with cooking spray in a large nonstick skillet over medium heat for a few seconds. Add the leeks, scallions, and onions; stir and lower heat.
Cook until the leeks and scallions start to brown, stirring occasionally. Add garlic and a few drops of water, the mixture will start to brown. Cook another few minutes, scraping the bottom of the pan so that nothing sticks.
Add the rest of the ingredients, except for the pecans, and cook for 3 minutes or until vegetables are tender.
Place on serving platter and sprinkle with pecan pieces.