You can substitute the same amount of broccoli for the broccoli rabe in this recipe. Cut florets into small pieces; peel and coarsely chop the thick stems.
In a large pot of boiling salted water, boil broccoli rabe 1 minute; with a slotted spoon, transfer to a colander to drain. In same pot of water, cook pasta until al dente, according to package instructions. Drain pasta, and return to pot.
Meanwhile, heat oil in a large skillet over medium heat. Add anchovy fillets (if using), garlic, and pepper flakes; cook, stirring, until fragrant, 30 to 60 seconds.
Add broccoli rabe; season with salt and pepper. Cook, stirring occasionally, until tender, 5 to 6 minutes. Stir in ricotta and milk; cook just until heated through, 3 to 4 minutes.
Add contents of skillet to pasta; season with salt and pepper, and toss to combine. Serve immediately.
When buying broccoli rabe, look for crisp, deep-green leaves and florets, with no signs of yellow or wilting.