In a Dutch oven over medium heat, melt the butter.
Add the garlic and onion and sauté for 1-2 minutes until fragrant and slightly softened.
Add the mushrooms and oil. Sauté for 2-3 minutes, until softened and browned.
Transfer the onions, garlic, and mushrooms to a bowl. Set aside.
Add the sausage to the Dutch oven. Cook, breaking it up with a wooden spoon, until mostly cooked through, about 5-6 minutes. Do not drain the grease.
Add the chicken broth to the pot and use a wooden spoon to scrape up the browned bits from the bottom.
Add the salt, pepper, thyme, and sage. Stir to combine.
Return the mushroom mixture to the pot. Stir in the lemon juice.
Add the kale to the pot. Reduce the heat to low, cover, and simmer 20–25 minutes, until the kale is tender and the flavors are blended.
Taste and adjust seasoning as needed. Serve hot with crushed red pepper garnish, if desired.