Trim and discard the tough green parts from the leeks, then cut them in in half lengthwise. Swirl in water to remove any sand. Pat dry.
On a large baking tray, place the leeks, sweet potatoes and garlic. Pour melted butter and salt evenly over the vegetables.
Bake for 25 minutes.
In a large saucepan, bring the white wine, thyme and parsley to a boil and cook until it loses the raw alcohol smell, about 5 minutes. Reduce the heat to a bare simmer.
When the leeks are golden take both leeks and garlic off the tray. Place potatoes back in the oven for another 20 minutes.
Add leeks and garlic into the simmering wine sauce. Cook until leeks are tender, about 30 minutes.
Melt 2 tablespoons butter in a nonstick skillet over medium-high heat. Pull the leeks from the cooking liquid, pat them dry and add to the butter to cook briefly on both sides, about 3 minutes total.
Remove the stem ends from the leeks and cut into 2-inch (5 cm) lengths.
For each taco, heat 2 tortillas on a griddle or in a large skillet over medium-high heat with a little butter. Place 2 slices of Oaxaca cheese next to them on the griddle. When the tortillas are warmed through, stack them on top of the cheese.
When the cheese has browned on one side, remove the tortillas and cheese to a plate, cheese-side up.
Add 2 to 3 strips of leek and some sweet potato. Spoon over 1½ to 2 teaspoons salsa and sprinkle with 2 teaspoons crumbled feta, a few toasted pine nuts, and 1 teaspoon minced green onions and serve immediately. Repeat using the remaining ingredients.
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