Heat olive oil in a large skillet over medium-high heat. Add fennel and onion; cover and cook for 4 to 5 minutes, stirring occasionally, until the onion is softened.
Cook potatoes for 3 to 4 minutes in a large pot of boiling water. When potatoes are ready, using a slotted spoon, transfer them to the skillet along with ½ cup of the boiling water and the Ranch seasoning; cook, covered, stirring occasionally until potatoes are tender about 10 minutes more.
Add pasta to boiling potato water. Reduce heat and cook according to package directions. Drain pasta, reserving ½ cup of water.
Toss pasta in skillet with vegetables adding additional reserved water, if needed. Garnish with fennel fronds.