Preheat the oven to 450F or 425F convection. Line a baking sheet with parchment. Add enough oil to lightly coat the parchment (about 2 tablespoons) and spread it around (this will help the burgers get crispy!).
To cook the lentils, add them to a medium pot and fill with enough water to cover by 2 inches. Bring to a boil over high heat, then reduce to medium-low to maintain a rapid simmer for 12-15 minutes, or until cooked through. Drain thoroughly, then transfer to a large bowl to cool off for a few minutes.
Once cool enough to handle, use your hands to squeeze the lentils into more of a paste, with some whole lentils still remaining. Add the herbs, oat flour, sun dried tomatoes, miso, egg, oregano, salt, pepper, and garlic. Mix again with your hands until thoroughly combined.
Use a rough ⅓ measuring cup to scoop 6 mounds onto the baking sheet and form them into large patties (about ½-inch thick). Brush some more oil on top of each burger (this helps them get nice and crispy without having to pan-fry them!). Bake for 12-15 minutes, or until dark golden brown and crispy on the bottom and tops.
To make feta yogurt sauce (if using), add all ingredients in a small bowl and use a fork to mash and mix the feta into the yogurt. You can thin the sauce out with a little bit of water or lemon if you want a runnier sauce.
Assemble your burgers with your desired toppings.