Step 1

1 Bring a large pot of water to a boil over high heat

Step 2

2 Add a generous pinch of salt and the pasta, and cook according to the package directions until al dente

Step 3

3 Reserve 1 cup of the cooking water and drain

Step 4

4 Rinse and dry the watercress; you should have at least 1 1/4 cups

Step 5

5 Discard the oil from the tuna

Step 6

6 Finely grate the zest of 1/2 to 1 orange to yield 1 1/2 to 2 teaspoons, letting it fall into a large bowl

Step 7

7 Cut that orange and the other(s) in half; squeeze them to yield 3/4 cup of juice

Step 8

8 Add 1/2 cup of the juice to the bowl, along with the olive oil, vinegar, mustard and crushed red pepper flakes, whisking to combine

Step 9

9 Add the watercress and the drained pasta, tossing to combine and coat evenly

Step 10

10 Taste, and add some or all of the remaining orange juice, as needed; if the orange flavor is too prominent, use some of the reserved pasta cooking water to dilute the sauce

Step 11

11 Scatter the drained tuna over the pasta

Step 12

12 Sprinkle with a small pinch of crushed red pepper flakes, if desired

Step 13

13 Serve right away, family-style, at the table