Bring a large pot of lightly salted water to a rolling boil.
Add the shell pasta to the boiling water and cook until al dente, about 10 to 11 minutes. Drain well.
In a large mixing bowl, combine the thawed peas, diced Cheddar cheese, diced ham, and chopped onion.
Whisk together the mayonnaise, vegetable oil, lemon juice, salt, and pepper in a small bowl.
Pour the dressing over the salad ingredients, add the drained pasta, and toss to coat evenly. Cover the bowl with plastic wrap and refrigerate for 2 to 3 hours to allow flavors to meld.