Step 1In a medium bowl, whisk together the shallots, garlic, lemon zest, lemon juice, agave syrup, salt and pepper, then slowly whisk in the olive oil to emulsify. (You can also use a small food processor to make this dressing by putting all ingredients into the processor and blending until emulsifed.) This makes a generous cup of of salad dressing, which will keep, refrigerated in an airtight container, up to 3 days.
Step 1Heat the oven to 350 degrees. Toast the walnuts in a shallow baking pan until fragrant and golden brown, about 10 minutes.
Step 2Place the unpeeled beets in a large saucepan and cover with water. Bring the water to a boil, then reduce the heat to a gentle simmer, cover and cook until the beets can easily be pierced through with a fork, about 30 minutes. Set the beets aside until cool enough to handle, then peel and slice into one-fourth-inch thick rounds. Place the beets in a small bowl (separate bowls if using different colors to keep the colors from bleeding), and toss with vinaigrette to coat.
Step 3Place the greens on individual plates or on a large shallow serving bowl. Arrange the sliced beets on top of the salad greens. Sprinkle the fennel and red onion slices on top of the beets, then drizzle over the vinaigrette before serving.