Add the chopped walnuts to a skillet and place over medium-high heat on the stove, stirring occasionally, until fragrant. It should take at least two minutes. Transfer the toasted walnuts to a dish and wipe out the pan.
Add the chopped pancetta to the pan and saute over medium-high heat, stirring frequently, until browned. It should take around six minutes. Use a slotted spoon to transfer the pancetta to a plate lined with paper towels. Blot the pancetta with a paper towel to absorb the grease.
Chop around ¼ of a red onion until you have ⅓ cup of chopped onions.
Drain 12 ounces of mandarin oranges and set aside.
Place your spinach in a large serving bowl. Add the walnuts, pancetta, red onions and mandarin oranges.
For the orange vinaigrette, rinse and dry off a large orange. Zest the orange and set the zest aside. Cut the orange in half.
In a liquid measuring cup, squeeze half an orange until you have ¼ cup of orange juice. (Use another orange half if needed.)
Add ¼ cup olive oil to the measuring cup. Add the vinegar, zest, salt, garlic powder and pepper. Whisk to combine.
Mix the dressing with the spinach in your bowl immediately before serving. Store leftovers in the refrigerator for up to four days.
Feel free to omit the pancetta for a vegan option. This spinach salad is still delicious without it!This makes 8 side-dish servings.