Combine clementine juice, vinegar, olive oil, Dijon, sugar, and pepper in a container with a tight-fitting lid and shake vigorously.
Heat a small skillet over medium-low heat and add the almonds. Cook and stir the almonds until lightly toasted. Remove from pan to a small plate to cool.
Toss the lettuce, spinach, clementines, toasted almonds, and feta in a large bowl with the vinaigrette. Serve.