Add the olive oil to a large skillet over medium heat. Once hot, add the onion and peas. Season with salt and pepper to taste. Sauté, stirring occasionally, until the vegetables soften slightly, about 3-4 minutes. Once softened, add the sun-dried tomatoes and spinach to the skillet. Cook for 1 to 2 more minutes, stirring occasionally, until the sun-dried tomatoes soften and the spinach wilts.