Thinly slice yellow onions.

Heat a small pot over medium-high, add 2 tbsp of olive oil. Add the yellow onion slices, stirring every 2 – 3 minutes.

After 5 – 10 minutes, once the heat is strong and the onions start to brown, turn the heat to medium-low. Stir every 2 – 3 minutes, allowing the onions to evenly caramelize. This should take about 20 minutes. 

Transfer onions to a small bowl. Set aside. 

Peel the sweet potatoes and thinly slice using a sharp knife or mandolin. Set into a large bowl.

Add the olive oil, salt, pepper, garlic powder, and umami powder into the bowl and mix until evenly coated. 

Core and thinly slice the gala apple, set aside.

Arrange the thin slices of sweet potato in the casserole dish length-wise. 

In groups of 4 or 5 pieces, add gala apple slices sporadically into the sweet potato arrangement. 

Bake for 30 minutes, until slightly soft.

As the sweet potatoes bake, prepare the toasted walnut crumble.

Finely chop the walnuts & slices of bread separately until it is pea-sized or resembles coarse sand.

Transfer the chopped walnuts & bread into a medium-sized bowl. 

Add the olive oil, salt, and pepper. Mix until combined.

Transfer the crumble to a pan over medium-high heat, toasting until fragrant and slightly golden. This may take about 6 – 8 minutes. 

Again, place the crumble back into a bowl. Set aside.

After the sweet potatoes have baked for 30 minutes, remove them from the oven and evenly spread the caramelized onion and butter on top. 

Bake for another 20 minutes to allow the juices to soak in. 

Remove from oven again, adding the toasted walnut crumble and drizzling the maple syrup over the top.

Bake for another 10 minutes.

Garnish with fresh thyme or desired herbs.

Serve immediately.