Heat the olive oil in a large heavy-bottomed pot over medium heat.
Add the diced onion and sauté for 5 minutes until translucent and softened.
Add the minced garlic, dried oregano, and red pepper flakes; cook for 1 minute until fragrant.
Pour in the vegetable broth and increase heat to medium-high to bring the liquid to a gentle simmer.
Add the cheese tortellini to the simmering broth and cook according to package instructions, approximately 4 to 5 minutes.
Stir in the chopped kale and cook for 2 minutes until the leaves are wilted and bright green.
Remove the pot from the heat.
Whisk in the grated Parmesan cheese and lemon juice until the cheese is incorporated into the broth.
Season with salt and black pepper to taste and serve immediately.