Walnut Lentil Burgers - Half Cup Habit

Ingredients

• 3/4 cup lentils, brown (or green; uncooked)
• 1 1/2 cups water
• 2 tbsps cider vinegar
• 1 tbsp olive oil (extra virgin; plus more for frying the burgers, as needed)
• 1 cup onion (finely minced)
• 2 tsps garlic (minced)
• 1 tsp mustard (dry)
• 10 mushrooms ((about 2-inch diameter), minced)
• 3/4 tsp salt
• 1/2 pound spinach (baby; finely minced (optional))
• pepper (black; freshly ground (to taste))
• 1/2 cup walnuts (California; finely minced)
• 1 cup breadcrumbs (whole wheat (from toasted whole-wheat sandwich bread))
• cheddar cheese (grated; for melting on top (optional))