Heat oil in a large pot or Dutch oven over medium. Add onion, turnips, 1/2 teaspoon salt, and 1/2 teaspoon black pepper; cook, stirring occasionally, until starting to brown, 4 to 6 minutes. Add celery, diced jalapeños, garlic, bay leaves, harissa, smoked paprika, cumin, and thyme; cook, stirring constantly, until fragrant, 1 to 2 minutes. Add peas, stock, ham hock (if using), 1 1/2 teaspoons salt, and remaining 1 teaspoon black pepper; bring to a simmer over medium-high. Reduce heat to medium-low; cover and cook, stirring occasionally, until peas are tender, about 45 minutes. Remove from heat; stir in turnip greens until just wilted. Let stand 15 minutes.