Preheat the oven to 425 degrees F and line a small baking pan with parchment paper.
Remove stems from mushrooms and place mushrooms upside down. Bake in oven for 15-20 minutes. The mushrooms will release a lot of water. Remove mushrooms from pan, draining the water onto the baking sheet. Set mushrooms aside and the pour off all the water in the baking sheet.
While mushrooms are baking, in a skillet over medium heat, heat the olive oil and add garlic, sliced onions and jalapeno pepper. Cook for 5 minutes until onions are soft and translucent.
Add the diced tomatoes and chopped cilantro. Stir to combine. Add the baby spinach and kale mixture, stir to combine, cover, and let simmer for 2-4 minutes or until just wilted. Season with sea salt and fresh cracked black pepper.
Reduce oven heat to 375 degrees. Fill the portobello mushrooms with the stuffing mixture. Top with grated cheddar cheese and bake in oven for 10 minutes or until cheese is melted.
Serve with your favorite sides or salad.