Thinly slice the yellow onion and add to a small skillet with the olive oil over medium-low heat.
Cook the onions for 10-15 minutes, stirring occasionally, until they are soft and deep golden brown.
Season the caramelized onions with sea salt and black pepper, then remove from heat.
Lightly butter one side of each slice of sprouted grain bread with the grass-fed butter.
Place one slice of bread butter-side down in a clean skillet over medium heat.
Layer half of the cheddar cheese, half of the turkey breast, and half of the caramelized onions onto the bread.
Place the second slice of bread on top, then layer the remaining cheese, turkey, and onions.
Top with the third slice of bread, butter-side facing up.
Grill for 3-4 minutes per side, pressing down slightly with a spatula, until the bread is golden brown and the cheese is fully melted.
Slice diagonally and serve immediately while hot.