Preheat the oven to 450°F. Trim the leaves from the cauliflower and remove 1 inch off the bottom of the stem. Carefully cut the cauliflower into 4 steaks approximately 1" thick. Transfer them to a baking sheet and brush with 1 tbsp vegetable oil. Season both sides with oregano, salt, and pepper. Roast until crisp in places and well browned about 20 to 24 minutes.
Add the shallot, celery, and parsley leaves to a medium bowl with the juice from half the lemon, and 1 tbsp olive oil. Season with salt and toss to coat.
Drain the roasted red peppers and add them to a blender. Add the garlic, half the pepitas, remaining lemon juice, 2 tbsp olive oil, 1 tbsp water, and a pinch of salt and pepper. Blend on high, scraping down the sides as necessary, until smooth.
In a nonstick skillet over medium heat and add the remaining pepitas. Cook, tossing occasionally, until toasted and fragrant, about 3 to 5 minutes. Transfer the toasted pepitas to a small bowl.
Drain, rinse, and pat the butter beans dry with paper towels. Return the skillet to medium-high heat with 1 tbsp olive oil. Once it’s hot, add the butter beans and a pinch of salt and pepper cook, shaking the pan occasionally, until crisp in places, about 2 to 4 minutes.
Spread the red pepper sauce on large plates and top with crispy butter beans and the cauliflower steaks. Top with the celery salad and sprinkle with toasted pepitas.