Add cannellini beans, garlic, rosemary, olive oil, lemon juice, salt, and pepper to a food processor. Blend until smooth, scraping down sides as needed. Transfer to a bowl, cover, and refrigerate until ready to use. (Can be made up to 3 days ahead.)

Place dried porcini in a small bowl and cover with ½ cup boiling water. Let soak for 15 minutes. Remove porcini, squeezing excess liquid back into bowl. Finely chop the porcini. Strain the soaking liquid through a fine-mesh sieve or coffee filter to remove grit. Reserve liquid.

Working in batches, pulse cremini mushrooms in a food processor until finely chopped (pea-sized pieces). Don't over-process into a paste.

Heat 2 tablespoons olive oil in a large skillet over medium-high heat until shimmering. Add half the chopped cremini mushrooms and cook without stirring for 2 minutes to develop color. Stir and continue cooking until mushrooms release their liquid and the pan is completely dry, 8-10 minutes. Transfer to a large bowl. Repeat with remaining 1 tablespoon oil and remaining cremini mushrooms.

Return all mushrooms to the skillet over medium heat. Add onion and a pinch of salt. Cook, stirring occasionally, until onion is soft and translucent, 5-7 minutes.

Add garlic, chopped porcini, thyme, and rosemary. Cook until fragrant, about 1 minute.

Add sherry and reserved porcini soaking liquid. Cook, stirring, until liquid is completely evaporated, 2-3 minutes.

Stir in soy sauce, tomato paste, balsamic vinegar, salt, and pepper. Cook for 1 minute.

Remove from heat. Stir in cooked lentils and toasted walnuts. Taste and adjust seasoning—the filling should be deeply savory. Let cool completely, at least 30 minutes. (Can be made up to 2 days ahead and refrigerated.)

Preheat oven to 400°F. Line a baking sheet with parchment paper.

Brush portobello caps on both sides with olive oil and season with salt and pepper. Place gill-side down on the prepared baking sheet.

Roast for 15 minutes until tender and lightly browned. Transfer to a plate lined with paper towels, gill-side down, to drain excess moisture. Let cool completely. (Can be made up to 1 day ahead and refrigerated.)

Line a rimmed baking sheet with parchment paper. Lay the thawed puff pastry sheet on a lightly floured surface. If needed, roll gently to create a rectangle approximately 12 x 14 inches.

Spread the white bean mixture in an even layer down the center third of the pastry, leaving a 2-inch border at the top and bottom. The spread should cover an area roughly 4 inches wide and 10 inches long.

Spoon half of the lentil-mushroom filling over the white bean spread, pressing gently to compact.

Arrange the portobello caps in a single row down the center, overlapping slightly if needed. Spoon the remaining lentil-mushroom filling over and around the portobellos, pressing to enclose them completely.

Fold the top and bottom edges of the pastry up and over the ends of the filling. Brush the edges with plant milk.

Fold one long side of the pastry over the filling, then fold the other side over, overlapping by about 1 inch. Press gently to seal. Carefully flip the Wellington seam-side down onto the prepared baking sheet.

Whisk together plant milk and olive oil. Brush the entire surface of the pastry with this mixture.

Using a sharp knife, score the top of the pastry with diagonal lines about 1 inch apart, cutting only about ⅛ inch deep (don't cut through to the filling). Repeat in the opposite direction to create a crosshatch pattern. Sprinkle with flaky sea salt.

Refrigerate for 15-20 minutes to chill the pastry. (This helps it puff evenly.)

Bake at 400°F for 35-40 minutes, until the pastry is deep golden brown and crisp. If the top browns too quickly, tent loosely with foil.

Let rest for 10 minutes before slicing with a sharp serrated knife. Serve with mushroom gravy.